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.

electron.vite.config.ts

@@ -1,6 +1,7 @@
 import { resolve } from 'path'
 import { defineConfig } from 'electron-vite'
 import react from '@vitejs/plugin-react'
+import tailwindcss from '@tailwindcss/vite'
 
 export default defineConfig({
   main: {},
@@ -11,6 +12,6 @@ export default defineConfig({
         '@renderer': resolve('src/renderer/src')
       }
     },
-    plugins: [react()]
+    plugins: [react(), tailwindcss()]
   }
 })