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/Settings/PlayerControls/PanelPageEditorView.swift

@@ -15,7 +15,6 @@ struct PanelPageEditorView: View {
 
     // Local state for immediate UI updates
     @State private var isEnabled: Bool = true
-    @State private var region: PanelRegion = .primary
     @State private var showDescription: Bool = true
     @State private var descriptionExpandedByDefault: Bool = false
     @State private var descriptionCollapsedLines: Int = InfoPageSettings.defaultDescriptionCollapsedLines
@@ -47,10 +46,6 @@ struct PanelPageEditorView: View {
                 enabledSection
             }
 
-            if PanelSettingsView.supportsCornerLayout, viewModel.panelLayoutMode == .corner {
-                regionSection
-            }
-
             switch pageType {
             case .info:
                 infoElementsSection
@@ -101,28 +96,6 @@ struct PanelPageEditorView: View {
         }
     }
 
-    // MARK: - Region Section (corner layout)
-
-    private var regionSection: some View {
-        Section {
-            Picker(
-                String(localized: "panel.settings.page.region"),
-                selection: $region
-            ) {
-                ForEach(PanelRegion.allCases, id: \.self) { region in
-                    Text(region.displayName).tag(region)
-                }
-            }
-            .disabled(!viewModel.canEditActivePreset)
-            .onChange(of: region) { _, newValue in
-                guard newValue != configuration?.region else { return }
-                viewModel.setPanelPageRegion(newValue, for: pageType)
-            }
-        } footer: {
-            Text(String(localized: "panel.settings.page.region.footer"))
-        }
-    }
-
     // MARK: - Info Elements Section
 
     @ViewBuilder
@@ -351,7 +324,6 @@ struct PanelPageEditorView: View {
 
     private func syncLocalState() {
         isEnabled = configuration?.isEnabled ?? true
-        region = configuration?.region ?? .primary
         let info = infoSettings
         showDescription = info.effectiveShowDescription
         descriptionExpandedByDefault = info.effectiveDescriptionExpandedByDefault