feat: tabbed profile page with PIN setup and auto employee numbers

- Profile page split into Account, Security, Appearance tabs
- Security tab: change password + set/change/remove POS PIN
- Warning banner with link to Security tab when no PIN is set
- /auth/me returns employeeNumber and hasPin
- Migration 0046: Postgres trigger auto-assigns sequential employee
  numbers starting at 1001, backfills existing users
- Added shadcn Alert component

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

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

@@ -323,6 +323,13 @@
       "when": 1775770000000,
       "tag": "0045_registers-reports",
       "breakpoints": true
+    },
+    {
+      "idx": 46,
+      "version": "7",
+      "when": 1775860000000,
+      "tag": "0046_auto-employee-number",
+      "breakpoints": true
     }
   ]
 }