Add corner player layout on iPad and macOS

Preset-selected two-region layout: video in the top-left, a resizable
side column, and an optional region below the video. Pages are assigned
to regions per preset (Location picker in the page editor); the layout
falls back to the stacked widescreen layout below 900pt width, so Split
View and narrow windows degrade automatically.

- CornerPlayerLayout hosts the shared playerAreaForWidescreen builder,
  so the MPV hosting path is identical to the stacked layout
- Layout resolver in ExpandedPlayerSheet picks corner per layout pass;
  NavigationCoordinator.isCornerLayoutActive feeds the macOS window
  manager, which drops the aspect-ratio lock and auto-fit while corner
  is active (and restores them on exit, incl. fullscreen replay and
  window re-mounts)
- Side column width persisted via new cornerPanelColumnWidth setting
  (280-560pt, video column kept >= 480pt)
- Settings: layout mode picker (iPad/macOS) with automatic-fallback
  footer, per-page region picker when corner is on, region shown in
  the pages list

YatteeTests/PanelSettingsTests.swift

@@ -160,4 +160,18 @@ struct PanelSettingsTests {
         settings.setEnabled(true, for: .comments)
         #expect(settings.pages.map(\.pageType) == PanelSettings.default.pages.map(\.pageType))
     }
+
+    @Test("Region assignment and layout mode round-trip through Codable")
+    func regionAndLayoutModeRoundTrip() throws {
+        var settings = PanelSettings.default
+        settings.layoutMode = .corner
+        settings.setRegion(.belowVideo, for: .related)
+        #expect(settings.configuration(for: .related)?.region == .belowVideo)
+        #expect(settings.enabledPages(in: .primary).map(\.pageType) == [.chapters, .queue])
+
+        let data = try JSONEncoder().encode(settings)
+        let decoded = try JSONDecoder().decode(PanelSettings.self, from: data)
+        #expect(decoded.effectiveLayoutMode == .corner)
+        #expect(decoded.configuration(for: .related)?.region == .belowVideo)
+    }
 }