feat: add PIN-based login with server-side cookie sessions

- Server: POST /auth/login (verify PIN, set httpOnly cookie)
- Server: GET /auth/check, POST /auth/logout
- Client: Login page shown when not authenticated
- Session persists 7 days via httpOnly cookie
- PIN stored server-side only (ADMIN_PIN env var)
- Dashboard loads data only after successful auth

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/locales/en.json

@@ -168,6 +168,13 @@
     "namePlaceholder": "e.g. Ramadan",
     "add": "Add"
   },
+  "login": {
+    "subtitle": "Enter your PIN to access the dashboard",
+    "placeholder": "PIN code",
+    "submit": "Login",
+    "invalid": "Invalid PIN code",
+    "error": "Connection error. Please try again."
+  },
   "errors": {
     "config": "The dashboard is not configured. Please set up the ERP API connection.",
     "network": "Cannot reach the database server. Please check your internet connection.",