web: vite + react + tailwind hello page with dark-mode toggle

Workspace packages export TS source under a 'source' condition used by
vite, node (--conditions=source) and tsc.

Task: M0-T4

apps/web/package.json

@@ -10,9 +10,17 @@
     "typecheck": "tsc --noEmit"
   },
   "dependencies": {
-    "@selfphoneng/shared": "workspace:*"
+    "@selfphoneng/shared": "workspace:*",
+    "react": "^19.3.1",
+    "react-dom": "^19.3.1",
+    "react-router": "^8.4.0"
   },
   "devDependencies": {
+    "@tailwindcss/vite": "^4.3.3",
+    "@types/react": "^19.3.0",
+    "@types/react-dom": "^19.3.0",
+    "@vitejs/plugin-react": "^6.1.2",
+    "tailwindcss": "^4.3.3",
     "vite": "^8.3.4"
   }
 }