feat(youtube): queue media for background cache and fill fields when rea

- Add `youtube.mediaCache.maxHeight` config option (default 720p)
- Background mode creates text-only cards while cache downloads, queues media updates, fills audio/image fields once cached file is ready
- Announce cache download start and readiness via overlay/OSD notifications
- Skip mpv stream indexes when generating audio from a YouTube cached file

src/anki-integration/pending-youtube-media.ts

@@ -0,0 +1,53 @@
+export interface PendingYoutubeMediaUpdate {
+  sourceUrl: string;
+  noteId: number;
+  startTime: number;
+  endTime: number;
+  label: string | number;
+  audioFieldName?: string;
+  imageFieldName?: string;
+  miscInfoFieldName?: string;
+  generateAudio: boolean;
+  generateImage: boolean;
+}
+
+function trimToNonEmptyString(value: unknown): string | null {
+  if (typeof value !== 'string') {
+    return null;
+  }
+  const trimmed = value.trim();
+  return trimmed.length > 0 ? trimmed : null;
+}
+
+function getYoutubeVideoId(rawUrl: string): string | null {
+  try {
+    const parsed = new URL(rawUrl);
+    const host = parsed.hostname.toLowerCase();
+    if (host === 'youtu.be' || host.endsWith('.youtu.be')) {
+      return trimToNonEmptyString(parsed.pathname.replace(/^\/+/, '').split('/')[0]);
+    }
+    if (host === 'youtube.com' || host.endsWith('.youtube.com')) {
+      const watchId = trimToNonEmptyString(parsed.searchParams.get('v'));
+      if (watchId) {
+        return watchId;
+      }
+      const parts = parsed.pathname.split('/').filter(Boolean);
+      if (parts[0] === 'shorts' || parts[0] === 'embed' || parts[0] === 'live') {
+        return trimToNonEmptyString(parts[1]);
+      }
+    }
+  } catch {
+    return null;
+  }
+  return null;
+}
+
+export function youtubeMediaUrlsMatch(a: string, b: string): boolean {
+  if (a.trim() === b.trim()) {
+    return true;
+  }
+
+  const aVideoId = getYoutubeVideoId(a);
+  const bVideoId = getYoutubeVideoId(b);
+  return Boolean(aVideoId && bVideoId && aVideoId === bVideoId);
+}