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/batches.py

@@ -1,4 +1,5 @@
 from fastapi import APIRouter, Depends, HTTPException
+from sqlalchemy import case
 from sqlalchemy.orm import Session, joinedload
 from typing import List
 
@@ -16,7 +17,7 @@ def list_batches(db: Session = Depends(get_db), current_user: User = Depends(get
         db.query(Batch)
         .options(joinedload(Batch.variety))
         .filter(Batch.user_id == current_user.id)
-        .order_by(Batch.sow_date.desc().nullslast(), Batch.created_at.desc())
+        .order_by(case((Batch.sow_date.is_(None), 1), else_=0), Batch.sow_date.desc(), Batch.created_at.desc())
         .all()
     )