feat: add keyframe index extraction and frame/keyframe navigation

Wire ffprobe-based keyframe indexing through Tauri and auto-load positions after download completes.

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

src/lib/stores/videoSession.svelte.ts

@@ -1,6 +1,7 @@
 import type { VideoMetadata } from '$lib/bindings/video';
 import { startDownload } from '$lib/bindings/video';
 import type { ThumbnailSpritesheet } from '$lib/bindings/mediaAnalysis';
+import { extractKeyframes } from '$lib/bindings/mediaAnalysis';
 
 export type SessionStatus = 'idle' | 'resolving' | 'ready' | 'error';
 export type DownloadStatus = 'idle' | 'downloading' | 'complete' | 'failed';
@@ -77,6 +78,15 @@ export function reset() {
   session.error = null;
 }
 
+async function triggerPostDownloadProcessing(filePath: string) {
+  try {
+    session.keyframePositions = await extractKeyframes(filePath);
+  } catch (e) {
+    console.error('Keyframe extraction failed:', e);
+  }
+  // Waveform and thumbnails will be added in Tasks 10 and 11
+}
+
 export async function beginDownload() {
   if (!session.url) return;
 
@@ -98,6 +108,7 @@ export async function beginDownload() {
           session.downloadProgress = 1.0;
           session.localFilePath = path;
           session.downloadStatus = 'complete';
+          void triggerPostDownloadProcessing(path);
         } else {
           session.downloadStatus = 'failed';
         }