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/Views/Player/CornerPlayerLayout.swift

@@ -134,30 +134,23 @@ 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.
+    /// Whether the info page is assigned below the video — the below panel
+    /// only exists to host it. (`effectiveRegion` already clamps to what
+    /// this platform can render; info is always enabled.)
     private var hasBelowVideoInfo: Bool {
-        guard let info = panelSettings.configuration(for: .info) else { return false }
-        return info.isEnabled && info.region == .belowVideo
+        panelSettings.effectiveRegion(for: .info) == .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.
+    /// comments pill (iOS only, and only together with the info panel — the
+    /// pill lives inside it; `effectiveRegion` enforces both).
     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
+        panelSettings.isPageEnabled(.comments)
+            && panelSettings.effectiveRegion(for: .comments) == .belowVideo
     }
 
-    /// 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.
+    /// Pages the below-video host renders; everything else goes to the side
+    /// column.
     private var belowVideoPageTypes: Set<PanelPageType> {
         var types: Set<PanelPageType> = []
         if hasBelowVideoInfo { types.insert(.info) }