fix: about dialog, cache paths, keyframes, reprocess always uses low-res

- Replace native macOS About menu with custom Svelte dialog via Tauri event
- Inline SVG icon in AboutDialog (fixes broken image in webview)
- Align cache directory with Tauri $APPDATA, add to asset protocol scope
- Fix ffprobe 8.x keyframe parsing (skip_frame nokey, flexible parser)
- Copy caption files into cache dir when outside cache path
- Add -y flag to ffmpeg thumbnail extraction (prevent hang on re-extract)
- Reprocess always uses preview (low-res) file, never HQ export
- Prefer cached preview file for analysis even when HQ is cached
- Guard upgrade toast when already on HQ

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

src-tauri/tauri.conf.json

@@ -27,7 +27,8 @@
           "/tmp/**",
           "/private/tmp/**",
           "/private/var/**",
-          "/var/**"
+          "/var/**",
+          "$APPDATA/**"
         ]
       }
     }