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

internal/handlers/auth.go

@@ -87,6 +87,7 @@ type UpdateProfileRequest struct {
 	FirstName string `json:"first_name,omitempty" validate:"omitempty,max=100"`
 	LastName  string `json:"last_name,omitempty" validate:"omitempty,max=100"`
 	Theme     string `json:"theme,omitempty" validate:"omitempty"`
+	Timezone  string `json:"timezone,omitempty" validate:"omitempty"`
 }
 
 type AdminUpdateUserRequest struct {
@@ -564,6 +565,22 @@ func (h *AuthHandler) UpdateProfile(c *echo.Context) error {
 		}
 	}
 
+	// Update timezone
+	if req.Timezone != "" {
+		if _, err := time.LoadLocation(req.Timezone); err != nil {
+			return c.JSON(http.StatusBadRequest, map[string]string{
+				"error": "Invalid timezone",
+			})
+		}
+		err := h.db.UpdateUserTimezone(ctx, database.UpdateUserTimezoneParams{
+			ID:       pgtype.UUID{Bytes: userUUID, Valid: true},
+			Timezone: pgtype.Text{String: req.Timezone, Valid: true},
+		})
+		if err != nil {
+			return err
+		}
+	}
+
 	// Update email (if provided)
 	if req.Email != "" {
 		existingUser, err := h.db.GetUserByEmail(c.Request().Context(), req.Email)