fix(web): preserve aw_url when a settings POST omits it

A UI save was silently blanking the configured AW URL because
handlePostSettings bound the whole Settings from the request and saved
it as-is. Now reads s.settings.AWURL under the existing settingsMu lock
and carries it forward when req.AWURL is empty, so fields not exposed in
the form are never erased.

internal/web/settings_handlers.go

@@ -45,6 +45,11 @@ func (s *Server) handlePostSettings(c *gin.Context) {
 	s.settingsMu.Lock()
 	apply := s.applyFn
 	path := s.settingsPath
+	// aw_url is not exposed in the settings form; preserve the persisted value
+	// so a UI save never blanks a configured AW URL.
+	if req.AWURL == "" {
+		req.AWURL = s.settings.AWURL
+	}
 	s.settingsMu.Unlock()
 	if apply == nil {
 		c.JSON(http.StatusInternalServerError, gin.H{"error": "settings not wired"})