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/App.svelte

@@ -14,6 +14,7 @@
   import { getSelectedClipId, removeClip } from '$lib/stores/clips.svelte';
   import { adjustShuttle, resetShuttleRate } from '$lib/transport/playback';
   import { session } from '$lib/stores/videoSession.svelte';
+  import { listen } from '@tauri-apps/api/event';
   import type { TransportKeyAction } from '$lib/transport/playback';
 
   let showSetupWizard = $state(true);
@@ -35,6 +36,14 @@
     loadPreferences();
   });
 
+  // Listen for native macOS menu "About" action
+  $effect(() => {
+    const unlisten = listen('open-about', () => {
+      showAboutDialog = true;
+    });
+    return () => { unlisten.then((fn) => fn()); };
+  });
+
   $effect(() => {
     if (session.status === 'resolving') {
       shuttleRate = 1;