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/mockScenarios.ts

@@ -0,0 +1,17 @@
+export type MockScenario = "rich" | "empty" | "busy" | "duplicates" | "album" | "errors" | "huge";
+
+const SCENARIOS = new Set<MockScenario>([
+  "rich",
+  "empty",
+  "busy",
+  "duplicates",
+  "album",
+  "errors",
+  "huge",
+]);
+
+export function getMockScenario(): MockScenario {
+  if (typeof window === "undefined") return "rich";
+  const value = new URLSearchParams(window.location.search).get("scenario");
+  return value && SCENARIOS.has(value as MockScenario) ? (value as MockScenario) : "rich";
+}