feat: migrate from React to Preact, add Vitest testing, and optimize bundle

Major framework and tooling changes:
- Migrate from React 19 to Preact 10 with preact-iso for routing
- Replace @mantine/hooks with custom hooks (useBooleanToggle, useViewportSize)
- Switch from @heroicons/react to lucide-preact for icons
- Replace chance.js with random-js for RNG

Testing infrastructure:
- Add Vitest with @testing-library/preact and happy-dom
- Set up test configuration and initial App tests
- Add Vitest UI for interactive test running

Build optimizations:
- Add cssnano for CSS minification
- Configure aggressive bundle optimizations in Vite
- Update to Tailwind CSS 4.1.17

Project structure:
- Consolidate entry point from main.tsx to index.tsx
- Reorganize CSS location (styles/index.css → index.css)
- Add ESLint with preact config
- Update TypeScript configuration for Preact

vitest.config.ts

@@ -0,0 +1,28 @@
+import { defineConfig } from "vitest/config";
+import preact from "@preact/preset-vite";
+
+export default defineConfig({
+  plugins: [preact()],
+  test: {
+    environment: "happy-dom",
+    globals: true,
+    setupFiles: ["./src/test/setup.ts"],
+    coverage: {
+      reporter: ["text", "json", "html"],
+      exclude: [
+        "node_modules/",
+        "src/test/",
+        "**/*.config.ts",
+        "**/*.d.ts",
+      ],
+    },
+  },
+  resolve: {
+    alias: {
+      "react": "preact/compat",
+      "react-dom": "preact/compat",
+      "react-dom/test-utils": "preact/test-utils",
+    },
+    dedupe: ["preact"],
+  },
+});