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/ViewModels/PlayerControls/PlayerControlsSettingsViewModel.swift

@@ -830,10 +830,15 @@ final class PlayerControlsSettingsViewModel {
         }
     }
 
-    /// Synchronously updates a page's corner-layout region assignment.
-    func setPanelPageRegion(_ region: PanelRegion, for pageType: PanelPageType) {
+    /// What the corner layout shows below the video (stored value).
+    var panelBelowVideoContent: PanelBelowVideoContent {
+        panelSettings.belowVideoContent
+    }
+
+    /// Synchronously updates what the corner layout shows below the video.
+    func syncPanelBelowVideoContent(_ content: PanelBelowVideoContent) {
         updatePanelSettingsSync { settings in
-            settings.setRegion(region, for: pageType)
+            settings.belowVideoContent = content
         }
     }