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/main/runtime/startup-osd-sequencer.ts

@@ -0,0 +1,106 @@
+export interface StartupOsdSequencerCharacterDictionaryEvent {
+  phase: 'checking' | 'generating' | 'syncing' | 'importing' | 'ready' | 'failed';
+  message: string;
+}
+
+export function createStartupOsdSequencer(deps: { showOsd: (message: string) => void }): {
+  reset: () => void;
+  markTokenizationReady: () => void;
+  showAnnotationLoading: (message: string) => void;
+  markAnnotationLoadingComplete: (message: string) => void;
+  notifyCharacterDictionaryStatus: (event: StartupOsdSequencerCharacterDictionaryEvent) => void;
+} {
+  let tokenizationReady = false;
+  let annotationLoadingMessage: string | null = null;
+  let pendingDictionaryProgress: StartupOsdSequencerCharacterDictionaryEvent | null = null;
+  let pendingDictionaryFailure: StartupOsdSequencerCharacterDictionaryEvent | null = null;
+  let dictionaryProgressShown = false;
+
+  const canShowDictionaryStatus = (): boolean =>
+    tokenizationReady && annotationLoadingMessage === null;
+
+  const flushBufferedDictionaryStatus = (): boolean => {
+    if (!canShowDictionaryStatus()) {
+      return false;
+    }
+    if (pendingDictionaryProgress) {
+      deps.showOsd(pendingDictionaryProgress.message);
+      dictionaryProgressShown = true;
+      return true;
+    }
+    if (pendingDictionaryFailure) {
+      deps.showOsd(pendingDictionaryFailure.message);
+      pendingDictionaryFailure = null;
+      dictionaryProgressShown = false;
+      return true;
+    }
+    return false;
+  };
+
+  return {
+    reset: () => {
+      tokenizationReady = false;
+      annotationLoadingMessage = null;
+      pendingDictionaryProgress = null;
+      pendingDictionaryFailure = null;
+      dictionaryProgressShown = false;
+    },
+    markTokenizationReady: () => {
+      tokenizationReady = true;
+      if (annotationLoadingMessage !== null) {
+        deps.showOsd(annotationLoadingMessage);
+        return;
+      }
+      flushBufferedDictionaryStatus();
+    },
+    showAnnotationLoading: (message) => {
+      annotationLoadingMessage = message;
+      if (tokenizationReady) {
+        deps.showOsd(message);
+      }
+    },
+    markAnnotationLoadingComplete: (message) => {
+      annotationLoadingMessage = null;
+      if (!tokenizationReady) {
+        return;
+      }
+      if (flushBufferedDictionaryStatus()) {
+        return;
+      }
+      deps.showOsd(message);
+    },
+    notifyCharacterDictionaryStatus: (event) => {
+      if (
+        event.phase === 'checking' ||
+        event.phase === 'generating' ||
+        event.phase === 'syncing' ||
+        event.phase === 'importing'
+      ) {
+        pendingDictionaryProgress = event;
+        pendingDictionaryFailure = null;
+        if (canShowDictionaryStatus()) {
+          deps.showOsd(event.message);
+          dictionaryProgressShown = true;
+        }
+        return;
+      }
+
+      pendingDictionaryProgress = null;
+      if (event.phase === 'failed') {
+        if (canShowDictionaryStatus()) {
+          deps.showOsd(event.message);
+        } else {
+          pendingDictionaryFailure = event;
+        }
+        dictionaryProgressShown = false;
+        return;
+      }
+
+      pendingDictionaryFailure = null;
+      if (canShowDictionaryStatus() && dictionaryProgressShown) {
+        deps.showOsd(event.message);
+      }
+      dictionaryProgressShown = false;
+    },
+  };
+}