feat: add FFmpeg status check and shutdown button

- FFmpeg check no longer blocks server startup
- Add /api/checkFFmpeg endpoint to query FFmpeg availability
- Add /api/shutdown endpoint to gracefully stop the server
- Add FFmpeg status indicator in web header
- Add shutdown button in web interface
- Move static files output to cmd/bilidown/static for embedding
- Update .gitignore to exclude cmd/bilidown/static

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -19,4 +19,5 @@ Thumbs.db
 backup/
 *.db
 download/
-bin/
+bin/
+cmd/bilidown/static