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/EditTaskModal.tsx

@@ -1,6 +1,7 @@
 import React, { useState, useEffect } from 'react';
 import axios from '../api/client';
 import { X, Calendar, Clock, RotateCw, Zap } from 'lucide-react';
+import { formatLocalDate } from '../utils/dateUtils';
 
 interface ScheduledTask {
   id: string;
@@ -79,7 +80,7 @@ const EditTaskModal: React.FC<EditTaskModalProps> = ({ task, isOpen, onClose, on
         setScheduleMode('onetime');
         if (task.run_at) {
           const date = new Date(task.run_at);
-          setRunAtDate(date.toISOString().split('T')[0]);
+          setRunAtDate(formatLocalDate(date));
           setRunAtTime(date.toTimeString().slice(0, 5));
         }
       } else if (task.schedule_type === 'INTERVAL') {

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 };

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();