feat: add thumbnail extraction and timeline display

Wire ffmpeg frame extraction into post-download processing and render
adaptive thumbnail strips on the timeline via the asset protocol.

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

src-tauri/src/commands/media_analysis.rs

@@ -1,4 +1,5 @@
-use crate::services::{keyframe_index, waveform_generator};
+use crate::models::ThumbnailSpritesheet;
+use crate::services::{keyframe_index, thumbnail_extractor, waveform_generator};
 
 #[tauri::command]
 pub async fn extract_keyframes(file_path: String) -> Result<Vec<f64>, String> {
@@ -18,3 +19,19 @@ pub async fn extract_waveform(
     .await
     .map_err(|e| format!("Task failed: {e}"))?
 }
+
+#[tauri::command]
+pub async fn extract_thumbnails(
+    file_path: String,
+    duration: f64,
+) -> Result<Vec<ThumbnailSpritesheet>, String> {
+    tokio::task::spawn_blocking(move || {
+        let temp_dir = std::env::temp_dir()
+            .join("video-clipper-thumbs")
+            .join(uuid::Uuid::new_v4().to_string());
+        let temp_dir_str = temp_dir.to_string_lossy().to_string();
+        thumbnail_extractor::extract_thumbnails(&file_path, duration, &temp_dir_str)
+    })
+    .await
+    .map_err(|e| format!("Task failed: {e}"))?
+}