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/current-media-tokenization-gate.ts

@@ -0,0 +1,70 @@
+function normalizeMediaPath(mediaPath: string | null | undefined): string | null {
+  if (typeof mediaPath !== 'string') {
+    return null;
+  }
+  const trimmed = mediaPath.trim();
+  return trimmed.length > 0 ? trimmed : null;
+}
+
+export function createCurrentMediaTokenizationGate(): {
+  updateCurrentMediaPath: (mediaPath: string | null | undefined) => void;
+  markReady: (mediaPath: string | null | undefined) => void;
+  waitUntilReady: (mediaPath: string | null | undefined) => Promise<void>;
+} {
+  let currentMediaPath: string | null = null;
+  let readyMediaPath: string | null = null;
+  let pendingMediaPath: string | null = null;
+  let pendingPromise: Promise<void> | null = null;
+  let resolvePending: (() => void) | null = null;
+
+  const resolvePendingWaiter = (): void => {
+    resolvePending?.();
+    resolvePending = null;
+    pendingPromise = null;
+    pendingMediaPath = null;
+  };
+
+  const ensurePendingPromise = (mediaPath: string): Promise<void> => {
+    if (pendingMediaPath === mediaPath && pendingPromise) {
+      return pendingPromise;
+    }
+    resolvePendingWaiter();
+    pendingMediaPath = mediaPath;
+    pendingPromise = new Promise<void>((resolve) => {
+      resolvePending = resolve;
+    });
+    return pendingPromise;
+  };
+
+  return {
+    updateCurrentMediaPath: (mediaPath) => {
+      const normalizedPath = normalizeMediaPath(mediaPath);
+      if (normalizedPath === currentMediaPath) {
+        return;
+      }
+      currentMediaPath = normalizedPath;
+      readyMediaPath = null;
+      resolvePendingWaiter();
+      if (normalizedPath) {
+        ensurePendingPromise(normalizedPath);
+      }
+    },
+    markReady: (mediaPath) => {
+      const normalizedPath = normalizeMediaPath(mediaPath);
+      if (!normalizedPath) {
+        return;
+      }
+      readyMediaPath = normalizedPath;
+      if (pendingMediaPath === normalizedPath) {
+        resolvePendingWaiter();
+      }
+    },
+    waitUntilReady: async (mediaPath) => {
+      const normalizedPath = normalizeMediaPath(mediaPath) ?? currentMediaPath;
+      if (!normalizedPath || readyMediaPath === normalizedPath) {
+        return;
+      }
+      await ensurePendingPromise(normalizedPath);
+    },
+  };
+}