chore(dev): add a dev-only demo panel for screenshotting transient UI

The background-worker pipeline drains in under a second on a fast machine,
making it impossible to screenshot live. This adds a dev-only panel
(Ctrl+Shift+D) that injects a frozen mid-pipeline state into mediaJobProgress
so the worker bar can be captured at leisure, then cleared. Gated by
import.meta.env.DEV and verified absent from the production bundle, so it
never ships. A first rough cut of the broader screenshot/demo mode.

src/App.tsx

@@ -12,6 +12,7 @@ import { TitleBar } from "./components/TitleBar";
 import { SettingsModal } from "./components/SettingsModal";
 import { UpdateToast } from "./components/UpdateToast";
 import { OnboardingOverlay } from "./components/onboarding/OnboardingOverlay";
+import { DemoPanel } from "./components/DemoPanel";
 import { initializeNotifications } from "./notifications";
 
 export default function App() {
@@ -94,6 +95,7 @@ export default function App() {
       <SettingsModal />
       <UpdateToast />
       <OnboardingOverlay />
+      {import.meta.env.DEV && <DemoPanel />}
     </div>
   );
 }