fix: implement timezone-aware datetime display and editing

All datetime values are stored in UTC but were displaying raw UTC times
to users, causing confusion (e.g., 10:00 Bucharest showing as 08:00).
This implements proper timezone conversion throughout the app using each
user's profile timezone setting.

Changes:
- Frontend: Replace local formatters with timezone-aware utilities
- Backend: Add timezone conversion to PUT /bookings endpoint
- FullCalendar: Configure to display events in user timezone
- Fix edit modal to preserve times when editing bookings

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

frontend/src/components/AttachmentsList.vue

@@ -42,8 +42,10 @@
 </template>
 
 <script setup lang="ts">
-import { ref, onMounted } from 'vue'
+import { ref, computed, onMounted } from 'vue'
 import { attachmentsApi, handleApiError } from '@/services/api'
+import { useAuthStore } from '@/stores/auth'
+import { formatDateTime as formatDateTimeUtil } from '@/utils/datetime'
 import type { Attachment } from '@/types'
 
 interface Props {
@@ -58,6 +60,8 @@ interface Emits {
 const props = defineProps<Props>()
 const emit = defineEmits<Emits>()
 
+const authStore = useAuthStore()
+const userTimezone = computed(() => authStore.user?.timezone || 'UTC')
 const attachments = ref<Attachment[]>([])
 const loading = ref(true)
 const error = ref('')
@@ -106,8 +110,7 @@ const formatFileSize = (bytes: number): string => {
 }
 
 const formatDate = (dateString: string): string => {
-  const date = new Date(dateString)
-  return date.toLocaleDateString() + ' ' + date.toLocaleTimeString([], { hour: '2-digit', minute: '2-digit' })
+  return formatDateTimeUtil(dateString, userTimezone.value)
 }
 
 onMounted(() => {