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

Yattee/ViewModels/PlayerControls/PlayerControlsSettingsViewModel.swift

@@ -819,6 +819,25 @@ final class PlayerControlsSettingsViewModel {
         }
     }
 
+    /// Current panel layout mode (iPad/macOS).
+    var panelLayoutMode: PanelLayoutMode {
+        panelSettings.effectiveLayoutMode
+    }
+
+    /// Synchronously updates the panel layout mode.
+    func syncPanelLayoutMode(_ mode: PanelLayoutMode) {
+        updatePanelSettingsSync { settings in
+            settings.layoutMode = mode
+        }
+    }
+
+    /// Synchronously updates a page's corner-layout region assignment.
+    func setPanelPageRegion(_ region: PanelRegion, for pageType: PanelPageType) {
+        updatePanelSettingsSync { settings in
+            settings.setRegion(region, for: pageType)
+        }
+    }
+
     /// Enables or disables a panel page. The last enabled page cannot be
     /// disabled — the panel always needs at least one page.
     func setPanelPageEnabled(_ pageType: PanelPageType, isEnabled: Bool) {