feat: add per-space timezone settings and improve booking management

- Add timezone configuration per space with fallback to system default
- Implement timezone-aware datetime display and editing across frontend
- Add migration for per_space_settings table
- Update booking service to handle timezone conversions properly
- Improve .gitignore to exclude build artifacts
- Add comprehensive testing documentation

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

frontend/src/utils/datetime.ts

@@ -2,6 +2,22 @@
  * Utility functions for timezone-aware datetime formatting.
  */
 
+/**
+ * Ensure datetime string has UTC marker (Z suffix).
+ * Backend returns naive datetimes without Z, which JS interprets as local time.
+ * This function adds Z to treat them as UTC.
+ */
+export const ensureUTC = (datetime: string): string => {
+  if (!datetime) return datetime
+  if (datetime.endsWith('Z')) return datetime
+  const tIndex = datetime.indexOf('T')
+  if (tIndex !== -1) {
+    const timePart = datetime.substring(tIndex + 1)
+    if (timePart.includes('+') || timePart.lastIndexOf('-') > 0) return datetime
+  }
+  return datetime + 'Z'
+}
+
 /**
  * Format a datetime string in the user's timezone.
  *
@@ -15,7 +31,7 @@ export const formatDateTime = (
   timezone: string = 'UTC',
   options?: Intl.DateTimeFormatOptions
 ): string => {
-  const date = new Date(datetime)
+  const date = new Date(ensureUTC(datetime))
 
   const defaultOptions: Intl.DateTimeFormatOptions = {
     timeZone: timezone,
@@ -47,7 +63,7 @@ export const formatDate = (datetime: string, timezone: string = 'UTC'): string =
  * Format time only in user's timezone.
  */
 export const formatTime = (datetime: string, timezone: string = 'UTC'): string => {
-  const date = new Date(datetime)
+  const date = new Date(ensureUTC(datetime))
   return new Intl.DateTimeFormat('ro-RO', {
     timeZone: timezone,
     hour: '2-digit',
@@ -59,7 +75,7 @@ export const formatTime = (datetime: string, timezone: string = 'UTC'): string =
  * Format datetime with timezone abbreviation.
  */
 export const formatDateTimeWithTZ = (datetime: string, timezone: string = 'UTC'): string => {
-  const date = new Date(datetime)
+  const date = new Date(ensureUTC(datetime))
 
   const formatted = new Intl.DateTimeFormat('ro-RO', {
     timeZone: timezone,
@@ -103,7 +119,7 @@ export const localDateTimeToISO = (localDateTime: string): string => {
  * Convert ISO datetime to datetime-local format for input field.
  */
 export const isoToLocalDateTime = (isoDateTime: string, timezone: string = 'UTC'): string => {
-  const date = new Date(isoDateTime)
+  const date = new Date(ensureUTC(isoDateTime))
 
   // Get the date components in the user's timezone
   const year = date.toLocaleString('en-US', { timeZone: timezone, year: 'numeric' })