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-tauri/src/commands/media_analysis.rs

@@ -1 +1,8 @@
-// Keyframe, waveform, thumbnail extraction commands — implemented in Tasks 9-11
+use crate::services::keyframe_index;
+
+#[tauri::command]
+pub async fn extract_keyframes(file_path: String) -> Result<Vec<f64>, String> {
+    tokio::task::spawn_blocking(move || keyframe_index::extract_keyframes(&file_path))
+        .await
+        .map_err(|e| format!("Task failed: {e}"))?
+}