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/overlay-runtime-options.ts

@@ -37,6 +37,10 @@ type OverlayRuntimeOptions = {
     data: KikuFieldGroupingRequestData,
   ) => Promise<KikuFieldGroupingChoice>;
   getKnownWordCacheStatePath: () => string;
+  getCachedMediaPath?: (
+    currentVideoPath: string,
+    kind: 'audio' | 'video',
+  ) => Promise<string | null>;
   shouldStartAnkiIntegration: () => boolean;
   bindOverlayOwner?: () => void;
   releaseOverlayOwner?: () => void;
@@ -71,6 +75,10 @@ export function createBuildInitializeOverlayRuntimeOptionsHandler(deps: {
     data: KikuFieldGroupingRequestData,
   ) => Promise<KikuFieldGroupingChoice>;
   getKnownWordCacheStatePath: () => string;
+  getCachedMediaPath?: (
+    currentVideoPath: string,
+    kind: 'audio' | 'video',
+  ) => Promise<string | null>;
   shouldStartAnkiIntegration: () => boolean;
   bindOverlayOwner?: () => void;
   releaseOverlayOwner?: () => void;
@@ -97,6 +105,7 @@ export function createBuildInitializeOverlayRuntimeOptionsHandler(deps: {
     showOverlayNotification: deps.showOverlayNotification,
     createFieldGroupingCallback: deps.createFieldGroupingCallback,
     getKnownWordCacheStatePath: deps.getKnownWordCacheStatePath,
+    ...(deps.getCachedMediaPath ? { getCachedMediaPath: deps.getCachedMediaPath } : {}),
     shouldStartAnkiIntegration: deps.shouldStartAnkiIntegration,
     bindOverlayOwner: deps.bindOverlayOwner,
     releaseOverlayOwner: deps.releaseOverlayOwner,