fix: address coderabbit review findings

src/main/runtime/subtitle-prefetch-source.ts

@@ -0,0 +1,34 @@
+import { fileURLToPath } from 'node:url';
+
+export function getActiveExternalSubtitleSource(
+  trackListRaw: unknown,
+  sidRaw: unknown,
+): string | null {
+  if (!Array.isArray(trackListRaw) || sidRaw == null) {
+    return null;
+  }
+
+  const sid =
+    typeof sidRaw === 'number' ? sidRaw : typeof sidRaw === 'string' ? Number(sidRaw) : null;
+  if (sid == null || !Number.isFinite(sid)) {
+    return null;
+  }
+
+  const activeTrack = trackListRaw.find((entry: unknown) => {
+    if (!entry || typeof entry !== 'object') {
+      return false;
+    }
+    const track = entry as Record<string, unknown>;
+    return track.type === 'sub' && track.id === sid && track.external === true;
+  }) as Record<string, unknown> | undefined;
+
+  const externalFilename =
+    typeof activeTrack?.['external-filename'] === 'string'
+      ? activeTrack['external-filename'].trim()
+      : '';
+  return externalFilename || null;
+}
+
+export function resolveSubtitleSourcePath(source: string): string {
+  return source.startsWith('file://') ? fileURLToPath(new URL(source)) : source;
+}