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/src/commands/cache.rs

@@ -82,12 +82,25 @@ pub async fn save_analysis_to_cache(
         })
         .collect();
 
-    // Convert absolute caption path to relative
+    // Convert absolute caption path to relative (copy into cache if needed)
     let relative_caption = caption_file_path.and_then(|abs| {
-        std::path::Path::new(&abs)
-            .strip_prefix(&cache_dir)
-            .ok()
-            .map(|rel| rel.to_string_lossy().to_string())
+        let abs_path = std::path::Path::new(&abs);
+        // Already inside cache dir
+        if let Ok(rel) = abs_path.strip_prefix(&cache_dir) {
+            return Some(rel.to_string_lossy().to_string());
+        }
+        // Outside cache dir — copy it in
+        if abs_path.is_file() {
+            let captions_dir = cache_dir.join("captions");
+            std::fs::create_dir_all(&captions_dir).ok();
+            if let Some(filename) = abs_path.file_name() {
+                let dest = captions_dir.join(filename);
+                if std::fs::copy(abs_path, &dest).is_ok() {
+                    return Some(format!("captions/{}", filename.to_string_lossy()));
+                }
+            }
+        }
+        None
     });
 
     let now = format!(