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/lib/components/AboutDialog.svelte

@@ -31,7 +31,43 @@
     onclick={(e) => e.stopPropagation()}
     onkeydown={(e) => e.stopPropagation()}
   >
-    <img class="app-icon" src="/favicon.svg" alt="GUI Video Clipper icon" />
+    <svg class="app-icon" viewBox="0 0 512 512" xmlns="http://www.w3.org/2000/svg">
+      <defs>
+        <linearGradient id="blade-grad" gradientUnits="userSpaceOnUse" x1="256" y1="130" x2="256" y2="290">
+          <stop offset="0%" stop-color="#fb923c"/>
+          <stop offset="100%" stop-color="#ea580c"/>
+        </linearGradient>
+      </defs>
+      <rect width="512" height="512" rx="90" fill="#18181b"/>
+      <g transform="translate(-3, 3)">
+        <polygon points="72,220 265,220 239,292 72,292" fill="#3f3f46"/>
+        <rect x="92" y="226" width="11" height="13" rx="3" fill="#27272a"/>
+        <rect x="130" y="226" width="11" height="13" rx="3" fill="#27272a"/>
+        <rect x="168" y="226" width="11" height="13" rx="3" fill="#27272a"/>
+        <rect x="92" y="273" width="11" height="13" rx="3" fill="#27272a"/>
+        <rect x="130" y="273" width="11" height="13" rx="3" fill="#27272a"/>
+        <rect x="168" y="273" width="11" height="13" rx="3" fill="#27272a"/>
+      </g>
+      <g transform="translate(3, -3)">
+        <polygon points="273,220 440,220 440,292 247,292" fill="#3f3f46"/>
+        <rect x="298" y="226" width="11" height="13" rx="3" fill="#27272a"/>
+        <rect x="345" y="226" width="11" height="13" rx="3" fill="#27272a"/>
+        <rect x="392" y="226" width="11" height="13" rx="3" fill="#27272a"/>
+        <rect x="278" y="273" width="11" height="13" rx="3" fill="#27272a"/>
+        <rect x="330" y="273" width="11" height="13" rx="3" fill="#27272a"/>
+        <rect x="382" y="273" width="11" height="13" rx="3" fill="#27272a"/>
+      </g>
+      <line x1="269" y1="216" x2="243" y2="296" stroke="#f97316" stroke-width="2" opacity="0.6"/>
+      <g transform="rotate(20, 256, 256)">
+        <polygon points="264,250 172,155 248,262" fill="url(#blade-grad)"/>
+        <polygon points="248,250 340,155 264,262" fill="url(#blade-grad)"/>
+        <polygon points="248,250 194,330 202,336 264,262" fill="#52525b"/>
+        <polygon points="264,250 318,330 310,336 248,262" fill="#52525b"/>
+        <circle cx="192" cy="348" r="20" stroke="#a1a1aa" stroke-width="4" fill="#18181b"/>
+        <circle cx="320" cy="348" r="20" stroke="#a1a1aa" stroke-width="4" fill="#18181b"/>
+        <circle cx="256" cy="256" r="7" fill="#a1a1aa"/>
+      </g>
+    </svg>
     <h2>GUI Video Clipper</h2>
     {#if version}
       <span class="version">v{version}</span>

src/lib/stores/videoSession.svelte.ts

@@ -209,9 +209,13 @@ async function triggerPostDownloadProcessing(filePath: string) {
       }
       if (cached.captionFilePath) {
         session.captionFilePath = cached.captionFilePath;
+      } else {
+        // Captions weren't cached — re-extract
+        await loadCaptionsWithCache(filePath);
       }
       session.processingStep = 'done';
-      if (session.exportStatus === 'complete' && session.exportFilePath) {
+      if (session.exportStatus === 'complete' && session.exportFilePath
+          && session.activeVideoPath !== session.exportFilePath) {
         session.showUpgradeToast = true;
       }
       return;
@@ -272,7 +276,8 @@ async function triggerPostDownloadProcessing(filePath: string) {
     console.error('Failed to save analysis to cache:', err);
   }
 
-  if (session.exportStatus === 'complete' && session.exportFilePath) {
+  if (session.exportStatus === 'complete' && session.exportFilePath
+      && session.activeVideoPath !== session.exportFilePath) {
     session.showUpgradeToast = true;
   }
 }
@@ -319,8 +324,9 @@ export async function beginDownload() {
     session.previewProgress = 1.0;
     session.previewStatus = 'complete';
     session.activeVideoPath = exportCached;
-    // No upgrade toast — we're already at best quality
-    triggerPostDownloadProcessing(exportCached);
+    // Prefer preview file for analysis (much faster ffmpeg operations)
+    const previewForProcessing = await checkCachedDownload(session.title, 'preview').catch(() => null);
+    triggerPostDownloadProcessing(previewForProcessing || exportCached);
     return;
   }
 
@@ -403,7 +409,15 @@ export async function beginDownload() {
 /** Clear cache for the current video and re-extract all analysis data. */
 export async function reprocessCurrentVideo() {
   if (!session.url || !session.activeVideoPath) return;
-  const filePath = session.activeVideoPath;
+
+  // Always use the low-res preview file for processing (never the HQ export)
+  let processingPath: string | null = session.previewFilePath;
+  if (!processingPath || processingPath === session.exportFilePath) {
+    // Try to find a cached preview file on disk
+    processingPath = await checkCachedDownload(session.title, 'preview').catch(() => null);
+  }
+  // Last resort: fall back to whatever is active (should rarely happen)
+  const filePath = processingPath || session.activeVideoPath;
 
   // Clear cache
   try {