feat: sidebar nav (default), group member management, per-customer ticketing identity mapping

- Navigation: new per-user setting (default "side") renders the page links as a
  left sidebar while keeping theme/bell/avatar/logout in the top-right; falls
  back to a top bar under 760px or when set to "top". Profile selector added.
- Group conversations: record a creatorId; the creator gets a "Manage members"
  button to add/remove participants (new GET/POST/DELETE members endpoints,
  creator-only). Existing groups backfilled.
- Customer ticketing: admin can map each assigned consultant to their username
  in that customer's ticketing system. Per-customer mapping takes precedence
  over the consultant's global ticketingIdentities during sync.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

internal/http/web.go

@@ -22,6 +22,7 @@ type pageData struct {
 	Active       string // nav highlight key
 	User         *domain.User
 	DefaultTheme string
+	NavLayout    string
 	Narrow       bool
 	OIDCEnabled  bool
 	Error        string
@@ -81,6 +82,12 @@ func (s *Server) render(w http.ResponseWriter, r *http.Request, page string, dat
 			data.DefaultTheme = data.User.Settings.Theme
 		}
 	}
+	if data.NavLayout == "" {
+		data.NavLayout = "side" // sidebar by default
+		if data.User != nil && data.User.Settings.NavLayout == "top" {
+			data.NavLayout = "top"
+		}
+	}
 	w.Header().Set("Content-Type", "text/html; charset=utf-8")
 	if err := t.ExecuteTemplate(w, "layout", data); err != nil {
 		s.log.Error("execute template", "page", page, "err", err)