feat: add background download manager with progress reporting

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

src/lib/components/UrlInput.svelte

@@ -1,6 +1,12 @@
 <script lang="ts">
   import { resolveUrl } from '$lib/bindings/video';
-  import { session, setMetadata, setError, setResolving } from '$lib/stores/videoSession.svelte';
+  import {
+    session,
+    setMetadata,
+    setError,
+    setResolving,
+    beginDownload,
+  } from '$lib/stores/videoSession.svelte';
 
   let inputValue = $state('');
 
@@ -12,6 +18,7 @@
     try {
       const meta = await resolveUrl(trimmed);
       setMetadata(meta);
+      void beginDownload();
     } catch (e) {
       setError(String(e));
     }