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/bindings/mediaAnalysis.ts

@@ -1,3 +1,5 @@
+import { invoke } from '@tauri-apps/api/core';
+
 export interface ThumbnailSpritesheet {
   filePath: string;
   startIndex: number;
@@ -7,3 +9,7 @@ export interface ThumbnailSpritesheet {
   columns: number;
   intervalSeconds: number;
 }
+
+export async function extractKeyframes(filePath: string): Promise<number[]> {
+  return invoke<number[]>('extract_keyframes', { filePath });
+}