feat: add background download manager with progress reporting

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

src/lib/stores/videoSession.svelte.ts

@@ -1,7 +1,9 @@
 import type { VideoMetadata } from '$lib/bindings/video';
+import { startDownload } from '$lib/bindings/video';
 import type { ThumbnailSpritesheet } from '$lib/bindings/mediaAnalysis';
 
 export type SessionStatus = 'idle' | 'resolving' | 'ready' | 'error';
+export type DownloadStatus = 'idle' | 'downloading' | 'complete' | 'failed';
 
 export const session = $state({
   status: 'idle' as SessionStatus,
@@ -16,6 +18,7 @@ export const session = $state({
   isPlaying: false,
   localFilePath: null as string | null,
   downloadProgress: 0,
+  downloadStatus: 'idle' as DownloadStatus,
   keyframePositions: [] as number[],
   waveformPeaks: [] as number[],
   thumbnailSpritesheets: [] as ThumbnailSpritesheet[],
@@ -33,6 +36,7 @@ export function setMetadata(meta: VideoMetadata) {
 
   session.localFilePath = null;
   session.downloadProgress = 0;
+  session.downloadStatus = 'idle';
   session.keyframePositions = [];
   session.waveformPeaks = [];
   session.thumbnailSpritesheets = [];
@@ -49,6 +53,7 @@ function clearMediaFields() {
   session.isPlaying = false;
   session.localFilePath = null;
   session.downloadProgress = 0;
+  session.downloadStatus = 'idle';
   session.keyframePositions = [];
   session.waveformPeaks = [];
   session.thumbnailSpritesheets = [];
@@ -71,3 +76,39 @@ export function reset() {
   session.status = 'idle';
   session.error = null;
 }
+
+export async function beginDownload() {
+  if (!session.url) return;
+
+  session.downloadStatus = 'downloading';
+  session.downloadProgress = 0;
+  session.localFilePath = null;
+
+  try {
+    await startDownload(
+      session.url,
+      (percent) => {
+        session.downloadProgress = percent;
+      },
+      (path) => {
+        session.localFilePath = path;
+      },
+      (success, path) => {
+        if (success) {
+          session.downloadProgress = 1.0;
+          session.localFilePath = path;
+          session.downloadStatus = 'complete';
+        } else {
+          session.downloadStatus = 'failed';
+        }
+      },
+      (message) => {
+        session.downloadStatus = 'failed';
+        console.error('Download error:', message);
+      }
+    );
+  } catch (e) {
+    session.downloadStatus = 'failed';
+    console.error('Download failed:', e);
+  }
+}