feat: add download button to Pi videos list

- /api/local-video/file?download=1 serves file as attachment
- Download link (green DL button) added to each row in local-videos.js
- Dashcam USB files skipped: no persistent mount for direct serve;
  use Import then download from Pi Videos

Co-authored-by: Copilot <[email protected]>

app.py

@@ -146,7 +146,17 @@ def local_video_file():
         return jsonify(error=str(error)), 400
     except FileNotFoundError:
         return jsonify(error=f"Local video not found: {relative_path}"), 404
-    return send_file(path, conditional=True, as_attachment=False)
+    as_attachment = request.args.get("download") == "1"
+    return send_file(path, conditional=True, as_attachment=as_attachment)
+
+@app.route('/api/dash/file/download')
+def dash_file_download():
+    relative_path = request.args.get("path", "")
+    try:
+        path = app_dash.safe_video_path(app_dash.DEFAULT_MOUNT_POINT, relative_path)
+    except (ValueError, FileNotFoundError) as error:
+        return jsonify(error=str(error)), 400
+    return send_file(path, as_attachment=True)
 
 @app.route('/api/local-video/thumbnail')
 def local_video_thumbnail():