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>

mysql/init.sql

@@ -6,7 +6,8 @@ CREATE TABLE IF NOT EXISTS users (
     hashed_password VARCHAR(255) NOT NULL,
     is_admin BOOLEAN NOT NULL DEFAULT FALSE,
     is_disabled BOOLEAN NOT NULL DEFAULT FALSE,
-    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+    last_login_at TIMESTAMP NULL DEFAULT NULL
 );
 
 CREATE TABLE IF NOT EXISTS varieties (