Wire up Tailwind CSS 4, Vitest, and core app libraries

Add Tailwind (via Vite plugin), Vitest+RTL test setup, and the
Zustand/TanStack Query/Lucide libraries the architecture plan calls
for. Strip the electron-vite demo boilerplate (logo, IPC ping demo,
Versions component) in favor of a minimal placeholder screen.

Verified: typecheck, lint, and test suite all pass; pnpm dev boots
and serves the correct renderer HTML.

vitest.config.ts

@@ -0,0 +1,23 @@
+import { resolve } from 'path'
+import { defineConfig } from 'vitest/config'
+import react from '@vitejs/plugin-react'
+
+export default defineConfig({
+  plugins: [react()],
+  resolve: {
+    alias: {
+      '@renderer': resolve('src/renderer/src')
+    }
+  },
+  test: {
+    environment: 'jsdom',
+    globals: true,
+    setupFiles: ['./src/test/setup.ts'],
+    include: ['src/**/*.test.{ts,tsx}'],
+    coverage: {
+      provider: 'v8',
+      reporter: ['text', 'html'],
+      include: ['src/renderer/src/**/*.{ts,tsx}']
+    }
+  }
+})