feat(ux): UX16 — autofocus the URL field on launch

An App mount effect focuses #aerofetch-url when the Downloads tab is active
(double-rAF for paint), so a user can paste + type immediately instead of
clicking first.

typecheck + eslint + prettier green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/renderer/src/App.tsx

@@ -104,6 +104,16 @@ function App(): React.JSX.Element {
     useDownloads.getState().hydrate()
   }, [])
 
+  // UX16: focus the URL field on launch so the user can paste + type immediately
+  // (Downloads is the default tab). Double-rAF waits for paint, matching the command
+  // palette's "New download" focus.
+  useEffect(() => {
+    if (useNav.getState().tab !== 'downloads') return
+    requestAnimationFrame(() =>
+      requestAnimationFrame(() => document.getElementById('aerofetch-url')?.focus())
+    )
+  }, [])
+
   // Promote scheduled ('saved' + a due scheduledFor) items when their time arrives.
   // The queue is persisted (M4), so a scheduled item survives a quit and fires on the
   // next launch once its time has passed. A 15s tick is plenty for minute-granularity