feat: add thumbnail extraction and timeline display

Wire ffmpeg frame extraction into post-download processing and render
adaptive thumbnail strips on the timeline via the asset protocol.

Co-authored-by: Cursor <cursoragent@cursor.com>

src-tauri/tauri.conf.json

@@ -18,7 +18,11 @@
       }
     ],
     "security": {
-      "csp": null
+      "csp": null,
+      "assetProtocol": {
+        "enable": true,
+        "scope": ["$TEMP/**", "$TMP/**", "/tmp/**"]
+      }
     }
   },
   "bundle": {