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/0046_auto-employee-number.sql

@@ -0,0 +1,31 @@
+-- Auto-assign employee_number on user insert if not provided
+CREATE OR REPLACE FUNCTION assign_employee_number()
+RETURNS TRIGGER AS $$
+DECLARE next_num INT;
+BEGIN
+  IF NEW.employee_number IS NULL OR NEW.employee_number = '' THEN
+    SELECT COALESCE(MAX(employee_number::int), 1000) + 1
+    INTO next_num
+    FROM "user"
+    WHERE employee_number ~ '^\d+$';
+    NEW.employee_number := next_num::text;
+  END IF;
+  RETURN NEW;
+END;
+$$ LANGUAGE plpgsql;
+
+DROP TRIGGER IF EXISTS trg_assign_employee_number ON "user";
+CREATE TRIGGER trg_assign_employee_number
+  BEFORE INSERT ON "user"
+  FOR EACH ROW
+  EXECUTE FUNCTION assign_employee_number();
+
+-- Backfill any users missing an employee number
+DO $$ DECLARE r RECORD; num INT;
+BEGIN
+  SELECT COALESCE(MAX(employee_number::int), 1000) INTO num FROM "user" WHERE employee_number ~ '^\d+$';
+  FOR r IN (SELECT id FROM "user" WHERE employee_number IS NULL OR employee_number = '' ORDER BY created_at) LOOP
+    num := num + 1;
+    UPDATE "user" SET employee_number = num::text WHERE id = r.id;
+  END LOOP;
+END $$;