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>

web/vite.config.ts

@@ -9,7 +9,7 @@ export default defineConfig({
         host: '0.0.0.0'
     },
     build: {
-        outDir: '../build/static',
+        outDir: '../cmd/bilidown/static',
         emptyOutDir: true,
     },
     css: {