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/lib/mediaSrc.ts

@@ -0,0 +1,17 @@
+import { convertFileSrc } from "@tauri-apps/api/core";
+
+const MOCK_MEDIA_PREFIX = "mock://";
+
+export function mediaSrc(path: string | null): string | null {
+  if (!path) return null;
+
+  if (import.meta.env.MODE === "ui" && (path.startsWith("/") || path.startsWith("http"))) {
+    return path;
+  }
+
+  if (import.meta.env.MODE === "ui" && path.startsWith(MOCK_MEDIA_PREFIX)) {
+    return `/dev-media/${path.slice(MOCK_MEDIA_PREFIX.length)}`;
+  }
+
+  return convertFileSrc(path);
+}