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/YearlyBlockCalendar.tsx

@@ -14,6 +14,7 @@ import { useTranslation } from 'react-i18next';
 import { ChevronLeft, ChevronRight, CalendarDays, X } from 'lucide-react';
 import { BlockedDate, TimeBlockListItem } from '../../types';
 import { useBlockedDates, useTimeBlock } from '../../hooks/useTimeBlocks';
+import { formatLocalDate } from '../../utils/dateUtils';
 
 interface YearlyBlockCalendarProps {
   resourceId?: string;
@@ -134,7 +135,7 @@ const YearlyBlockCalendar: React.FC<YearlyBlockCalendarProps> = ({
                 return <div key={`empty-${i}`} className="aspect-square" />;
               }
 
-              const dateKey = day.toISOString().split('T')[0];
+              const dateKey = formatLocalDate(day);
               const blocks = blockedDateMap.get(dateKey) || [];
               const hasBlocks = blocks.length > 0;
               const isToday = new Date().toDateString() === day.toDateString();