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/config/definitions/shared.ts

@@ -50,6 +50,7 @@ export const SPECIAL_COMMANDS = {
 
 export const DEFAULT_KEYBINDINGS: NonNullable<ResolvedConfig['keybindings']> = [
   { key: 'Space', command: ['cycle', 'pause'] },
+  { key: 'KeyF', command: ['cycle', 'fullscreen'] },
   { key: 'KeyJ', command: ['cycle', 'sid'] },
   { key: 'Shift+KeyJ', command: ['cycle', 'secondary-sid'] },
   { key: 'ArrowRight', command: ['seek', 5] },