Route corner layout pages by region and harden page selection

The below-video panel now renders only when the info page is assigned
to that region; comments assigned below become the portrait panel's
pill (iOS), and every other below-video page falls back to the side
column instead of disappearing. Page selection snaps to the first
configured page when the current one is disabled or reordered away,
and the page bar highlight follows the actually rendered page. The
related tab stays visible with a spinner while video details load
instead of vanishing until they arrive.

Yattee/Views/Player/CornerPlayerLayout.swift

@@ -95,12 +95,42 @@ struct CornerPlayerLayout<PlayerContent: View>: View {
         playerControlsLayout.effectivePanelSettings
     }
 
+    /// Whether the info page is assigned to (and enabled in) the below-video
+    /// region — the below panel only exists to host it.
+    private var hasBelowVideoInfo: Bool {
+        guard let info = panelSettings.configuration(for: .info) else { return false }
+        return info.isEnabled && info.region == .belowVideo
+    }
+
+    /// Comments assigned below the video render as the portrait panel's
+    /// comments pill (iOS only, and only while the info panel is below —
+    /// the pill lives inside it). Everything else falls back to the column.
+    private var hasBelowVideoComments: Bool {
+        #if os(iOS)
+        guard hasBelowVideoInfo,
+              let comments = panelSettings.configuration(for: .comments) else { return false }
+        return comments.isEnabled && comments.region == .belowVideo
+        #else
+        return false
+        #endif
+    }
+
+    /// Pages the below-video host actually renders. Enabled pages assigned
+    /// `.belowVideo` that it can't host (chapters/related/queue/tracks, or
+    /// comments without the info panel) fall back to the side column instead
+    /// of disappearing.
+    private var belowVideoPageTypes: Set<PanelPageType> {
+        var types: Set<PanelPageType> = []
+        if hasBelowVideoInfo { types.insert(.info) }
+        if hasBelowVideoComments { types.insert(.comments) }
+        return types
+    }
+
     /// Column emptiness is decided at the config level: runtime-empty pages
     /// (no chapters, no related yet) are hidden inside the panel content, not
     /// by collapsing the column — layout jumps during loading would be jarring.
-    /// Info always renders below the video; the side column holds the rest.
     private var hasSideColumn: Bool {
-        panelSettings.enabledPages().contains { $0.pageType != .info }
+        panelSettings.enabledPages().contains { !belowVideoPageTypes.contains($0.pageType) }
     }
 
     /// The info page config drives the below-video panel's element settings.
@@ -132,7 +162,7 @@ struct CornerPlayerLayout<PlayerContent: View>: View {
             let height = max(1, geometry.size.height)
 
             let showsSideColumn = isPanelVisible && hasSideColumn
-            let showsBelowVideo = isPanelVisible && isBelowPanelVisible
+            let showsBelowVideo = isPanelVisible && isBelowPanelVisible && hasBelowVideoInfo
 
             let effectiveColumnWidth = clampColumnWidth(columnWidth, availableWidth: width)
             // Grabber sits on the column's leading edge, inside the column
@@ -229,7 +259,7 @@ struct CornerPlayerLayout<PlayerContent: View>: View {
                         // reveal) via belowPanelDragOffset counting down
                         // from full hide travel.
                         .simultaneousGesture(
-                            (isPanelVisible && !isBelowPanelVisible)
+                            (isPanelVisible && !isBelowPanelVisible && hasBelowVideoInfo)
                                 ? DragGesture()
                                     .onChanged { value in
                                         guard appEnvironment?.navigationCoordinator.isAdjustingPlayerSliders != true else { return }
@@ -347,6 +377,11 @@ struct CornerPlayerLayout<PlayerContent: View>: View {
                let visible = settingsManager?.landscapeDetailsPanelVisible {
                 isPanelVisible = visible
             }
+            // Default the side column to the first configured page (skipping
+            // pages hosted below the video).
+            if let first = panelSettings.enabledPages().first(where: { !belowVideoPageTypes.contains($0.pageType) })?.pageType {
+                primaryPage = first
+            }
         }
         .onChange(of: columnWidth) { _, newWidth in
             settingsManager?.cornerPanelColumnWidth = newWidth
@@ -376,7 +411,7 @@ struct CornerPlayerLayout<PlayerContent: View>: View {
                 playerControlsLayout: playerControlsLayout,
                 onFullscreen: onFullscreen,
                 pillsOverlayOpacity: 1,
-                hidesCommentsPill: true,
+                hidesCommentsPill: !hasBelowVideoComments,
                 inlineInfoSections: false,
                 panelCornerRadius: 0,
                 onDragChanged: { offset in
@@ -485,7 +520,8 @@ struct CornerPlayerLayout<PlayerContent: View>: View {
         }
     }
 
-    /// Side column with tabs for every enabled page except info.
+    /// Side column with tabs for every enabled page the below-video host
+    /// doesn't render.
     @ViewBuilder
     private func sideColumnPanel(video: Video) -> some View {
         PlayerPanelContentView(
@@ -502,7 +538,7 @@ struct CornerPlayerLayout<PlayerContent: View>: View {
                 bottomInset: 16
             ),
             pageBarTopPadding: 8,
-            excludedPages: [.info]
+            excludedPages: belowVideoPageTypes
         )
         .background(panelBackgroundColor.ignoresSafeArea(edges: .bottom))
     }