Add last login tracking, batch date auto-fill, and bug fixes

- Track last_login_at on User model, updated on every successful login
- Show last login date in admin panel user table
- Fix admin/garden date display (datetime strings already contain T separator)
- Fix My Garden Internal Server Error (MySQL does not support NULLS LAST syntax)
- Fix Log Batch infinite loop when user has zero varieties
- Auto-fill batch dates from today when creating a new batch, calculated
  from selected variety's week offsets (germination, greenhouse, garden)
- Update README with new features and batch date auto-fill formula table

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/routers/admin.py

@@ -23,7 +23,7 @@ def list_users(db: Session = Depends(get_db), _: User = Depends(get_admin_user))
     return [
         AdminUserOut(
             id=u.id, email=u.email, is_admin=u.is_admin, is_disabled=u.is_disabled,
-            created_at=u.created_at, **_user_stats(db, u.id)
+            created_at=u.created_at, last_login_at=u.last_login_at, **_user_stats(db, u.id)
         )
         for u in users
     ]