feat: POS PIN unlock with employee number + PIN auth

- Add employeeNumber and pinHash fields to users table
- POST /auth/pin-login: takes combined code (4-digit employee# + 4-digit PIN)
- POST /auth/set-pin: employee sets their own PIN (requires full auth)
- DELETE /auth/pin: remove PIN
- Lock screen with numpad, auto-submits on 8 digits, visual dot separator
- POS uses its own auth token separate from admin session
- Admin "POS" link clears admin session before navigating
- /pos route has no auth guard — lock screen is the auth
- API client uses POS token when available, admin token otherwise
- Auto-lock timer reads pos_lock_timeout from app_config (default 15 min)
- Lock button in POS top bar, shows current cashier name

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

packages/backend/src/db/migrations/meta/_journal.json

@@ -302,6 +302,13 @@
       "when": 1775590000000,
       "tag": "0042_drawer-adjustments",
       "breakpoints": true
+    },
+    {
+      "idx": 42,
+      "version": "7",
+      "when": 1775590000000,
+      "tag": "0042_user-pin",
+      "breakpoints": true
     }
   ]
 }