feat(subtitles): add local Japanese subtitle generation

- Generate Japanese SRTs through the overlay modal or launcher
- Support managed whisper.cpp models and optional dialogue detection

src/core/services/subtitle-generation-srt.ts

@@ -0,0 +1,7 @@
+export function formatTimestamp(milliseconds: number): string {
+  const rounded = Math.max(0, Math.round(milliseconds));
+  const hours = Math.floor(rounded / 3600000);
+  const minutes = Math.floor((rounded % 3600000) / 60000);
+  const seconds = Math.floor((rounded % 60000) / 1000);
+  return `${String(hours).padStart(2, '0')}:${String(minutes).padStart(2, '0')}:${String(seconds).padStart(2, '0')},${String(rounded % 1000).padStart(3, '0')}`;
+}