Fix storyboard downloads with yattee-server direct YouTube URLs

yattee-server returns direct YouTube CDN URLs in the storyboard `url`
and `templateUrl` fields instead of an Invidious-style VTT proxy path.
Two resulting issues:

- `Storyboard.directSheetURL` was replacing the whole `M$M` token with
  just the index, producing `.../0.jpg` (404) instead of `.../M0.jpg`.
  Replace `M$M` with `M\(index)` to preserve the literal `M` prefix;
  matching the full token also avoids clobbering `$M` sequences that
  may appear in `sigh=rs$...` query params.
- The download code fetched `proxyUrl` as if it were a WebVTT file;
  with yattee-server that downloads a JPEG that fails UTF-8 parsing.
  Skip the VTT round-trip when `proxyUrl` obviously points at an image.

Also align the on-disk filename with the local-playback template
(`sb_M$M.jpg` → `sb_M{N}.jpg`) so offline seek-bar previews resolve,
and add [Storyboard] debug logs at each decision point so future
failures can be diagnosed without guessing.

Yattee/Models/Storyboard.swift

@@ -99,7 +99,12 @@ struct Storyboard: Hashable, Sendable, Codable {
     /// - Returns: Direct URL for the sprite sheet, or nil if invalid
     func directSheetURL(for index: Int) -> URL? {
         guard index >= 0, index < storyboardCount else { return nil }
-        let urlString = templateUrl.replacingOccurrences(of: "M$M", with: "\(index)")
+        // YouTube storyboard filenames are `M{N}.jpg` and the templateUrl encodes the
+        // slot as `M$M`. The leading `M` is literal, so replace `M$M` with `M{index}`
+        // (not bare `\(index)`) — otherwise the file becomes `0.jpg` instead of `M0.jpg`
+        // and YouTube returns 404. Matching the full `M$M` token also avoids accidentally
+        // rewriting any `$M` that appears later in query params such as `sigh=rs$...`.
+        let urlString = templateUrl.replacingOccurrences(of: "M$M", with: "M\(index)")
         return URL(string: urlString)
     }
 

Yattee/Services/Downloads/DownloadManager+Assets.swift

@@ -12,6 +12,21 @@ import Foundation
 extension DownloadManager {
     // MARK: - Storyboard Download
 
+    /// Returns true if `proxyUrl` clearly points at a direct image (e.g. a YouTube CDN
+    /// `.jpg` URL returned by yattee-server) rather than a VTT proxy path.
+    /// In that case fetching it and trying to parse as WebVTT would be wasted bandwidth
+    /// and produces an empty URL list, so we skip straight to the templateUrl fallback.
+    static func proxyUrlLooksLikeImage(_ urlString: String) -> Bool {
+        // Strip the query string; common YouTube URLs carry huge `sqp` / `sigh` params.
+        let pathOnly = urlString.split(separator: "?", maxSplits: 1).first.map(String.init) ?? urlString
+        let lower = pathOnly.lowercased()
+        return lower.hasSuffix(".jpg")
+            || lower.hasSuffix(".jpeg")
+            || lower.hasSuffix(".png")
+            || lower.hasSuffix(".webp")
+    }
+
+
     /// Start downloading storyboard sprite sheets sequentially
     func startStoryboardDownload(downloadID: UUID) {
         guard let index = activeDownloads.firstIndex(where: { $0.id == downloadID }),
@@ -43,10 +58,29 @@ extension DownloadManager {
                 return
             }
 
-            // First, try to get VTT from proxy URL to extract actual image URLs
+            // Diagnostic: log the selected storyboard variant so we can tell
+            // which server shape we are dealing with (Invidious VTT vs yattee-server direct URLs).
+            let proxySample = storyboard.proxyUrl.map { String($0.prefix(120)) } ?? "<nil>"
+            let templateSample = String(storyboard.templateUrl.prefix(120))
+            LoggingService.shared.debug(
+                "[Storyboard] Starting download for \(videoID): \(storyboard.width)x\(storyboard.height), sheets=\(storyboard.storyboardCount)",
+                category: .downloads,
+                details: "proxyUrl=\(proxySample) templateUrl=\(templateSample)"
+            )
+
+            // First, try to get VTT from proxy URL to extract actual image URLs.
+            // Some backends (yattee-server after innertube switch) return a direct image
+            // URL in the `url` field instead of a VTT proxy path, so skip the VTT round-trip
+            // when the URL obviously points at an image resource.
             var imageURLs: [URL] = []
 
             if let proxyUrl = storyboard.proxyUrl {
+                if Self.proxyUrlLooksLikeImage(proxyUrl) {
+                    LoggingService.shared.debug(
+                        "[Storyboard] Skipping VTT fetch — proxyUrl looks like a direct image, using templateUrl fallback",
+                        category: .downloads
+                    )
+                } else {
                     // Construct absolute VTT URL
                     let vttURL: URL?
                     if proxyUrl.hasPrefix("http://") || proxyUrl.hasPrefix("https://") {
@@ -65,31 +99,65 @@ extension DownloadManager {
 
                     if let vttURL {
                         do {
-                        let (vttData, _) = try await URLSession.shared.data(from: vttURL)
+                            let (vttData, response) = try await URLSession.shared.data(from: vttURL)
+                            let status = (response as? HTTPURLResponse)?.statusCode ?? -1
                             imageURLs = parseVTTForImageURLs(vttData, baseURL: vttURL)
+                            LoggingService.shared.debug(
+                                "[Storyboard] VTT fetch OK (status=\(status), bytes=\(vttData.count)) parsed \(imageURLs.count) URLs",
+                                category: .downloads
+                            )
                         } catch {
-                        // VTT fetch failed, will fall back to direct URLs
+                            LoggingService.shared.debug(
+                                "[Storyboard] VTT fetch failed: \(error.localizedDescription) — will fall back to direct URLs",
+                                category: .downloads
+                            )
+                        }
+                    } else {
+                        LoggingService.shared.debug(
+                            "[Storyboard] Could not construct VTT URL from proxyUrl, falling back to direct URLs",
+                            category: .downloads
+                        )
                     }
                 }
             }
 
             // If VTT parsing failed, fall back to direct URLs (may not work if blocked)
             if imageURLs.isEmpty, storyboard.storyboardCount > 0 {
+                var nilCount = 0
                 for sheetIndex in 0..<storyboard.storyboardCount {
                     if let url = storyboard.directSheetURL(for: sheetIndex) {
                         imageURLs.append(url)
+                    } else {
+                        nilCount += 1
                     }
                 }
+                let firstSample = imageURLs.first.map { String($0.absoluteString.prefix(160)) } ?? "<none>"
+                LoggingService.shared.debug(
+                    "[Storyboard] directSheetURL fallback produced \(imageURLs.count)/\(storyboard.storyboardCount) URLs (nil: \(nilCount))",
+                    category: .downloads,
+                    details: "first=\(firstSample)"
+                )
             }
 
             let totalSheets = imageURLs.count
             var completedSheets = 0
 
+            if totalSheets == 0 {
+                LoggingService.shared.debug(
+                    "[Storyboard] No sheet URLs to download after VTT + fallback — will mark as failed",
+                    category: .downloads
+                )
+            }
+
             // Download each sprite sheet sequentially
             for (sheetIndex, sheetURL) in imageURLs.enumerated() {
                 guard !Task.isCancelled else { return }
 
-                let fileName = "sb_\(sheetIndex).jpg"
+                // Filename must match the `sb_M$M.jpg` template used by
+                // `Storyboard.localStoryboard(...)` after `M$M` → `M{index}` substitution
+                // in `Storyboard.directSheetURL(for:)`, otherwise local playback won't
+                // resolve the sheets.
+                let fileName = "sb_M\(sheetIndex).jpg"
                 let fileURL = storyboardDir.appendingPathComponent(fileName)
 
                 // Skip if already downloaded
@@ -104,6 +172,12 @@ extension DownloadManager {
 
                     guard let httpResponse = response as? HTTPURLResponse,
                           httpResponse.statusCode == 200 else {
+                        let status = (response as? HTTPURLResponse)?.statusCode ?? -1
+                        LoggingService.shared.debug(
+                            "[Storyboard] Sheet \(sheetIndex) skipped — HTTP \(status) (bytes=\(data.count))",
+                            category: .downloads,
+                            details: String(sheetURL.absoluteString.prefix(160))
+                        )
                         continue
                     }
 
@@ -111,6 +185,10 @@ extension DownloadManager {
 
                     // Verify it's actually an image
                     guard contentType.contains("image") || data.count > 50000 else {
+                        LoggingService.shared.debug(
+                            "[Storyboard] Sheet \(sheetIndex) skipped — unexpected content (type=\(contentType), bytes=\(data.count))",
+                            category: .downloads
+                        )
                         continue
                     }
 
@@ -119,12 +197,20 @@ extension DownloadManager {
                     updateStoryboardProgress(downloadID: downloadID, completed: completedSheets, total: totalSheets)
 
                 } catch {
-                    // Continue with next sheet - non-fatal
+                    LoggingService.shared.debug(
+                        "[Storyboard] Sheet \(sheetIndex) errored: \(error.localizedDescription)",
+                        category: .downloads,
+                        details: String(sheetURL.absoluteString.prefix(160))
+                    )
                 }
             }
 
             // Complete storyboard phase
             let success = completedSheets > 0
+            LoggingService.shared.debug(
+                "[Storyboard] Download phase finished: \(completedSheets)/\(totalSheets) sheets succeeded",
+                category: .downloads
+            )
             finalizeStoryboardDownload(
                 downloadID: downloadID,
                 storyboardDirName: storyboardDirName,