Add timezone backend support (handlers, utilities, user context)

- Add FormatInTimezone and FormatTimestamptzInTimezone helpers
  in templates/utils.go for timezone-aware time display
- Add Timezone field to templates.User struct
- Pass user timezone from DB to template context in helpers.go
- Add timezone update handling in auth.go UpdateProfile with
  validation via time.LoadLocation
- Add UpdateTimezoneSettings handler in system_settings.go for
  admin system-wide default timezone using UpdateSystemSetting

templates/types.go

@@ -16,6 +16,7 @@ type User struct {
 	LastName  string
 	CreatedAt time.Time
 	Token     string
+	Timezone  string
 }
 
 type PageData struct {