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/models.py

@@ -44,6 +44,7 @@ class User(Base):
     is_admin = Column(Boolean, default=False, nullable=False)
     is_disabled = Column(Boolean, default=False, nullable=False)
     created_at = Column(DateTime, server_default=func.now())
+    last_login_at = Column(DateTime, nullable=True)
 
     varieties = relationship("Variety", back_populates="user", cascade="all, delete-orphan")
     batches = relationship("Batch", back_populates="user", cascade="all, delete-orphan")