feat: Add timezone architecture for consistent date/time handling

- Create dateUtils.ts with helpers for UTC conversion and timezone display
- Add TimezoneSerializerMixin to include business_timezone in API responses
- Update GeneralSettings timezone dropdown with IANA identifiers
- Apply timezone mixin to Event, TimeBlock, and field mobile serializers
- Document timezone architecture in CLAUDE.md

All times stored in UTC, converted for display based on business timezone.
If business_timezone is null, uses user's local timezone.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/components/time-blocks/TimeBlockCreatorModal.tsx

@@ -41,6 +41,7 @@ import {
   Resource,
   TimeBlockListItem,
 } from '../../types';
+import { formatLocalDate } from '../../utils/dateUtils';
 
 // Preset block types
 const PRESETS = [
@@ -418,8 +419,8 @@ const TimeBlockCreatorModal: React.FC<TimeBlockCreatorModalProps> = ({
     if (recurrenceType === 'NONE') {
       if (selectedDates.length > 0) {
         const sorted = [...selectedDates].sort((a, b) => a.getTime() - b.getTime());
-        data.start_date = sorted[0].toISOString().split('T')[0];
-        data.end_date = sorted[sorted.length - 1].toISOString().split('T')[0];
+        data.start_date = formatLocalDate(sorted[0]);
+        data.end_date = formatLocalDate(sorted[sorted.length - 1]);
       }
     } else if (recurrenceType === 'WEEKLY') {
       data.recurrence_pattern = { days_of_week: daysOfWeek };