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/domain/user.go

@@ -54,6 +54,7 @@ type NotificationPrefs struct {
 
 type UserSettings struct {
 	Theme             string            `bson:"theme" json:"theme"`
+	NavLayout         string            `bson:"navLayout" json:"navLayout"` // "side" (default) | "top"
 	Notifications     NotificationPrefs `bson:"notifications" json:"notifications"`
 	LeaderboardOptOut bool              `bson:"leaderboardOptOut" json:"leaderboardOptOut"`
 }