Replace per-page corner regions with one "Below Video" picker

The corner layout's below-video host is the portrait info panel, not a
page host: it can render only Info, plus Comments as the panel's pill on
iOS. The page editor still offered "Below Video" for every page, so
assigning Related there was accepted, stored, listed as such, and then
silently rendered in the side column.

- The Layout section gains a "Below Video" picker in corner mode:
  Nothing / Info / Info and Comments (macOS offers the first two). It
  writes the info/comments regions; the per-page Location section and
  its strings are gone.
- PanelSettings.normalizedPages clamps regions to what the host can
  render (only info/comments below, comments only together with info),
  so older presets self-correct. effectiveRegion(for:) resolves the
  platform-specific placement and is used by both the pages list labels
  and CornerPlayerLayout, replacing its own region checks.
- The layout schematic drops the below-video block for "Nothing"; the
  corner-mode footer explains the constraint.
- Tests cover the picker round-trip and the clamping.

Yattee/Models/PlayerControls/PanelSettings.swift

@@ -72,11 +72,16 @@ enum PanelPageType: String, Codable, CaseIterable, Sendable {
 /// Region a page is assigned to in the corner layout.
 /// `primary` is the right column in corner mode and the only panel elsewhere;
 /// the assignment is ignored outside corner mode.
+///
+/// The below-video host is the portrait info panel, not a page host: only
+/// `.info` can live there, plus `.comments` as the panel's pill (iOS). Every
+/// other page is clamped to `.primary` by `PanelSettings.normalizedPages`, and
+/// settings expose the choice as one `PanelBelowVideoContent` picker.
 enum PanelRegion: String, Codable, CaseIterable, Sendable {
     case primary
     case belowVideo
 
-    /// Localized name for the settings picker.
+    /// Localized name for the pages list.
     var displayName: String {
         switch self {
         case .primary:
@@ -87,6 +92,36 @@ enum PanelRegion: String, Codable, CaseIterable, Sendable {
     }
 }
 
+/// What the corner layout shows below the video: the three layouts the
+/// below-video host can actually render. Stored as the info/comments page
+/// regions (see `PanelSettings.belowVideoContent`).
+enum PanelBelowVideoContent: String, CaseIterable, Sendable {
+    /// Video fills the left column; every page sits in the side column.
+    case nothing
+    /// The info panel below the video; comments join the side column.
+    case info
+    /// The info panel with the comments pill below the video (iOS only).
+    case infoAndComments
+
+    /// Localized name for the settings picker.
+    var displayName: String {
+        switch self {
+        case .nothing:
+            return String(localized: "panel.belowVideo.nothing")
+        case .info:
+            return String(localized: "panel.belowVideo.info")
+        case .infoAndComments:
+            return String(localized: "panel.belowVideo.infoAndComments")
+        }
+    }
+
+    /// Options this platform can render: the comments pill lives on the
+    /// iOS portrait panel only.
+    static var availableCases: [PanelBelowVideoContent] {
+        PanelSettings.supportsBelowVideoComments ? allCases : [.nothing, .info]
+    }
+}
+
 // MARK: - Layout Mode
 
 /// How the details panel is laid out on iPad/macOS.
@@ -484,10 +519,27 @@ struct PanelSettings: Codable, Hashable, Sendable {
     /// other pages or switched off. Its element toggles remain configurable.
     static let pinnedPageType: PanelPageType = .info
 
+    /// Page types the below-video host can render: the info panel itself and
+    /// comments as its pill. Other types assigned `.belowVideo` (older
+    /// presets) are clamped back to the side column.
+    static let belowVideoHostablePageTypes: Set<PanelPageType> = [.info, .comments]
+
+    /// Whether this platform renders comments below the video (the comments
+    /// pill on the portrait info panel exists on iOS only).
+    static var supportsBelowVideoComments: Bool {
+        #if os(iOS)
+        return true
+        #else
+        return false
+        #endif
+    }
+
     /// The pages list with any page types this app version knows about but
     /// the stored list is missing appended as disabled, so newly introduced
-    /// pages surface in settings after an update, and the pinned page forced
-    /// first and enabled (older presets may have stored it elsewhere).
+    /// pages surface in settings after an update, the pinned page forced
+    /// first and enabled (older presets may have stored it elsewhere), and
+    /// regions clamped to what the corner layout can host: only info and
+    /// comments may sit below the video, comments only together with info.
     var normalizedPages: [PanelPageConfiguration] {
         var result = pages
         let known = Set(pages.map(\.pageType))
@@ -500,9 +552,59 @@ struct PanelSettings: Codable, Hashable, Sendable {
             pinned.isEnabled = true
             result.insert(pinned, at: 0)
         }
+
+        let infoBelowVideo = result.contains { $0.pageType == .info && $0.region == .belowVideo }
+        for index in result.indices where result[index].region == .belowVideo {
+            let type = result[index].pageType
+            let hostable = Self.belowVideoHostablePageTypes.contains(type)
+                && (type != .comments || infoBelowVideo)
+            if !hostable {
+                result[index].region = .primary
+            }
+        }
         return result
     }
 
+    /// The stored below-video choice, derived from the info/comments regions.
+    /// Platform-agnostic so presets keep their value across devices; use
+    /// `effectiveBelowVideoContent` for what this device renders.
+    var belowVideoContent: PanelBelowVideoContent {
+        get {
+            guard configuration(for: .info)?.region == .belowVideo else { return .nothing }
+            return configuration(for: .comments)?.region == .belowVideo ? .infoAndComments : .info
+        }
+        set {
+            normalize()
+            let infoRegion: PanelRegion = newValue == .nothing ? .primary : .belowVideo
+            let commentsRegion: PanelRegion = newValue == .infoAndComments ? .belowVideo : .primary
+            for index in pages.indices {
+                switch pages[index].pageType {
+                case .info: pages[index].region = infoRegion
+                case .comments: pages[index].region = commentsRegion
+                default: pages[index].region = .primary
+                }
+            }
+        }
+    }
+
+    /// The below-video content this platform renders: comments fall back to
+    /// the side column where the pill does not exist.
+    var effectiveBelowVideoContent: PanelBelowVideoContent {
+        let stored = belowVideoContent
+        if stored == .infoAndComments, !Self.supportsBelowVideoComments { return .info }
+        return stored
+    }
+
+    /// Where the corner layout actually places a page on this platform.
+    func effectiveRegion(for pageType: PanelPageType) -> PanelRegion {
+        switch (pageType, effectiveBelowVideoContent) {
+        case (.info, .info), (.info, .infoAndComments), (.comments, .infoAndComments):
+            return .belowVideo
+        default:
+            return .primary
+        }
+    }
+
     /// Page types the info page can render as collapsible inline sections
     /// below the description (iPhone, and the stacked iPad portrait panel).
     static let inlineSectionTypes: Set<PanelPageType> = [.chapters, .related]
@@ -615,12 +717,6 @@ struct PanelSettings: Codable, Hashable, Sendable {
         pages[index].settings = newSettings
     }
 
-    mutating func setRegion(_ region: PanelRegion, for pageType: PanelPageType) {
-        normalize()
-        guard let index = pages.firstIndex(where: { $0.pageType == pageType }) else { return }
-        pages[index].region = region
-    }
-
     private enum CodingKeys: String, CodingKey {
         case pages
         case layoutMode