feat(youtube): add mediaCache mode and safer stream media extraction

- Add `youtube.mediaCache.mode` config option (`direct` | `background`)
- Resolve EDL stream URLs to single audio/video URLs before ffmpeg extraction
- Pass reconnect, user-agent, and safe headers to ffmpeg for remote streams
- Add background yt-dlp media cache with fallback to direct extraction
- Introduce `MediaInput` structured type replacing bare path strings

src/main/runtime/youtube-playback-runtime.ts

@@ -20,6 +20,7 @@ export type YoutubePlaybackRuntimeDeps = {
   launchWindowsMpv: (playbackUrl: string, args: string[]) => Promise<LaunchResult>;
   waitForYoutubeMpvConnected: (timeoutMs: number) => Promise<boolean>;
   prepareYoutubePlaybackInMpv: (request: { url: string }) => Promise<boolean>;
+  startYoutubeMediaCache?: (url: string) => void | Promise<void>;
   runYoutubePlaybackFlow: (request: {
     url: string;
     mode: NonNullable<CliArgs['youtubeMode']>;
@@ -126,6 +127,15 @@ export function createYoutubePlaybackRuntime(deps: YoutubePlaybackRuntimeDeps) {
       if (!mediaReady) {
         throw new Error('Timed out waiting for mpv to load the requested YouTube URL.');
       }
+      if (deps.startYoutubeMediaCache) {
+        void Promise.resolve(deps.startYoutubeMediaCache(request.url)).catch((error) => {
+          deps.logWarn(
+            `Failed to start YouTube media cache: ${
+              error instanceof Error ? error.message : String(error)
+            }`,
+          );
+        });
+      }
 
       await deps.runYoutubePlaybackFlow({
         url: request.url,