Remove timezone from User model and pass as request parameter

Timezone is now sent from the client on each request that needs it,
instead of storing it in the database. This simplifies the model and
allows timezone to be dynamic (useful for traveling users).

Changes:
- Remove timezone field from User entity
- Remove timezone from user registration
- GET /habits/today now requires ?timezone= query param
- Add migration to drop timezone column from database
- Update related tests

internal/i18n/locales/en.json

@@ -46,7 +46,9 @@
     "invalid_token_or_password": "Invalid or expired token, or password requirements not met",
     "failed_reset_password": "Failed to reset password",
     "failed_delete_user": "Failed to delete user",
-    "failed_get_stats": "Failed to get statistics"
+    "failed_get_stats": "Failed to get statistics",
+    "timezone_required": "Timezone is required",
+    "invalid_timezone": "Invalid timezone (must be a valid IANA timezone)"
   },
   "success": {
     "registration_with_verification": "Registration successful. Please check your email to verify your account.",