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/schemas.py

@@ -33,6 +33,7 @@ class AdminUserOut(BaseModel):
     is_admin: bool
     is_disabled: bool
     created_at: Optional[datetime]
+    last_login_at: Optional[datetime]
     variety_count: int
     batch_count: int