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"})

internal/web/settings_handlers_test.go

@@ -111,6 +111,37 @@ func TestPostSettingsInvalidJSONIs400(t *testing.T) {
 	}
 }
 
+func TestPostSettingsPreservesAWURL(t *testing.T) {
+	s, _ := newTestServer(t)
+	path := filepath.Join(t.TempDir(), "settings.json")
+	fa := &fakeApplier{}
+	cur := settings.Settings{AIBackend: "claude", AWURL: "http://aw.remote:9999"}
+	s.SetSettings(path, cur, fa.apply)
+	r := s.Router()
+
+	// POST a body that omits aw_url (mirrors what the real UI form sends).
+	body := `{"ai_backend":"claude","marvin_cmd":"","knowledge_path":""}`
+	w := post(t, r, "/settings", body)
+	if w.Code != http.StatusOK {
+		t.Fatalf("status = %d, want 200 (body %s)", w.Code, w.Body.String())
+	}
+	if fa.called != 1 {
+		t.Fatalf("applier called %d times, want 1", fa.called)
+	}
+	// (a) applyFn must have received the preserved AWURL.
+	if fa.last.AWURL != "http://aw.remote:9999" {
+		t.Errorf("applied AWURL = %q, want http://aw.remote:9999", fa.last.AWURL)
+	}
+	// (b) persisted file must also preserve the AWURL.
+	saved, err := settings.Load(path)
+	if err != nil {
+		t.Fatalf("settings file not loadable: %v", err)
+	}
+	if saved.AWURL != "http://aw.remote:9999" {
+		t.Errorf("saved AWURL = %q, want http://aw.remote:9999", saved.AWURL)
+	}
+}
+
 func TestBrowseListsDirsAndMarkdown(t *testing.T) {
 	dir := t.TempDir()
 	if err := os.Mkdir(filepath.Join(dir, "sub"), 0o755); err != nil {