Add timezone support for visits, transportation, and lodging

- Introduced TIMEZONES constant in models.py to store valid timezone options.
- Updated Visit, Transportation, and Lodging models to include timezone fields.
- Modified serializers to include timezone fields in VisitSerializer, TransportationSerializer, and LodgingSerializer.
- Enhanced DateRangeCollapse component to handle timezone selection and formatting.
- Implemented timezone formatting functions in LodgingCard and TransportationCard components.
- Updated LodgingModal and TransportationModal to bind timezone data.
- Added VALID_TIMEZONES to dateUtils for consistent timezone management across the application.

frontend/src/lib/components/LodgingModal.svelte

@@ -56,7 +56,8 @@
 			is_public: lodgingToEdit?.is_public || false,
 			collection: lodgingToEdit?.collection || collection.id,
 			created_at: lodgingToEdit?.created_at || '',
-			updated_at: lodgingToEdit?.updated_at || ''
+			updated_at: lodgingToEdit?.updated_at || '',
+			timezone: lodgingToEdit?.timezone || ''
 		};
 	}
 
@@ -303,6 +304,7 @@
 					type="lodging"
 					bind:utcStartDate={lodging.check_in}
 					bind:utcEndDate={lodging.check_out}
+					bind:selectedStartTimezone={lodging.timezone}
 				/>
 
 				<!-- Location Information -->