Improve startup dictionary sync UX and default playback keybindings

- Add default `f` fullscreen overlay binding and switch default AniSkip skip key to `Tab`
- Make character-dictionary auto-sync non-blocking at startup with tokenization gating for Yomitan mutations
- Add ordered startup OSD progress (checking/generating/updating/importing), refresh current subtitle on sync completion, and extend regression tests

src/core/services/overlay-visibility.ts

@@ -84,19 +84,7 @@ export function updateVisibleOverlayVisibility(args: {
     }
   }
 
-  if (args.isMacOSPlatform || args.isWindowsPlatform) {
-    mainWindow.hide();
-    args.syncOverlayShortcuts();
-    return;
-  }
-
-  const fallbackBounds = args.resolveFallbackBounds?.();
-  if (!fallbackBounds) return;
-
-  args.updateVisibleOverlayBounds(fallbackBounds);
-  args.syncPrimaryOverlayWindowLayer('visible');
-  showPassiveVisibleOverlay();
-  args.enforceOverlayLayerOrder();
+  mainWindow.hide();
   args.syncOverlayShortcuts();
 }