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/utils.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"net/url"
 	"strings"
+	"time"
 
 	"github.com/google/uuid"
 	"github.com/jackc/pgx/v5/pgtype"
@@ -197,3 +198,25 @@ func formatAlternateInfo(data []byte) string {
 
 	return strings.Join(parts, " ")
 }
+
+// FormatInTimezone formats a time.Time in the specified timezone as MM-DD-YYYY HH:MM
+func FormatInTimezone(t time.Time, timezone string) string {
+	if t.IsZero() {
+		return ""
+	}
+
+	loc, err := time.LoadLocation(timezone)
+	if err != nil {
+		loc = time.UTC
+	}
+
+	return t.In(loc).Format("01-02-2006 03:04 PM")
+}
+
+// FormatTimestamptzInTimezone formats a pgtype.Timestamptz in the specified timezone
+func FormatTimestamptzInTimezone(t pgtype.Timestamptz, timezone string) string {
+	if !t.Valid {
+		return ""
+	}
+	return FormatInTimezone(t.Time, timezone)
+}