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/dev/applyMockScenario.ts

@@ -0,0 +1,17 @@
+import { useGalleryStore } from "../store";
+import { getMockScenario } from "./mockScenarios";
+
+export function applyMockScenario() {
+  const scenario = getMockScenario();
+  const store = useGalleryStore.getState();
+
+  if (scenario === "album") {
+    const albumId = store.albums[0]?.id;
+    if (albumId !== undefined) store.viewAlbum(albumId);
+    return;
+  }
+
+  if (scenario === "duplicates") {
+    store.setView("duplicates");
+  }
+}