feat: add Phokus UI Lab

Add a dev-only Vite UI mode with Tauri API mocks, in-memory fixture scenarios, reusable media source handling, and documentation for browser-based visual testing.

src/main.tsx

@@ -1,10 +1,19 @@
 import React from "react";
 import ReactDOM from "react-dom/client";
-import App from "./App";
 import "./index.css";
 
-ReactDOM.createRoot(document.getElementById("root") as HTMLElement).render(
-  <React.StrictMode>
-    <App />
-  </React.StrictMode>,
-);
+async function bootstrap() {
+  if (import.meta.env.MODE === "ui") {
+    await import("./dev/setupMockTauri");
+  }
+
+  const { default: App } = await import("./App");
+
+  ReactDOM.createRoot(document.getElementById("root") as HTMLElement).render(
+    <React.StrictMode>
+      <App />
+    </React.StrictMode>,
+  );
+}
+
+void bootstrap();