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/setupMockTauri.ts

@@ -0,0 +1,11 @@
+import { mockConvertFileSrc, mockIPC, mockWindows } from "@tauri-apps/api/mocks";
+import { handleMockCommand } from "./mockBackend";
+
+mockWindows("main");
+mockConvertFileSrc("windows");
+
+mockIPC((cmd, payload) => handleMockCommand(cmd, payload), {
+  shouldMockEvents: true,
+});
+
+console.info("[Phokus UI Lab] Mock Tauri backend installed.");