test: add frontend test suite with 54 tests

- Install vitest, @testing-library/svelte, jsdom
- Configure vitest in vite.config.js with browser conditions for Svelte 5
- Pure function tests for api.js (artworkUrl, bundleUrl, setupBundleUrl)
- Router tests (push, replace hash manipulation)
- GameCard component tests (title, year, genre, cover, badge, click)
- Library page tests (loading, empty, game grid, year/genre filters)
- GameDetail page tests (loading, error, display, edit mode, badges)
- Play page tests (loading, not-ready, unsupported, booting, error)

frontend/vite.config.js

@@ -1,3 +1,4 @@
+/// <reference types="vitest" />
 import { defineConfig } from "vite";
 import { svelte } from "@sveltejs/vite-plugin-svelte";
 
@@ -15,4 +16,12 @@ export default defineConfig({
     outDir: "dist",
     emptyOutDir: true,
   },
+  resolve: {
+    conditions: process.env.VITEST ? ["browser", "module", "import"] : [],
+  },
+  test: {
+    environment: "jsdom",
+    globals: true,
+    include: ["src/**/*.test.js"],
+  },
 });