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/sync/manager.go

@@ -257,7 +257,12 @@ func (m *Manager) SyncCustomer(ctx context.Context, c *domain.Customer) error {
 		if err != nil {
 			continue // consultant deleted; reconcile will catch up
 		}
-		identity := ticketingIdentity(consultant, c.Ticketing.Type)
+		// Per-customer mapping (set by the admin on the customer) wins over
+		// the consultant's global identity; fall back to the global one.
+		identity := c.Ticketing.ConsultantIdentities[consultantID]
+		if identity == "" {
+			identity = ticketingIdentity(consultant, c.Ticketing.Type)
+		}
 		if identity == "" {
 			continue // §5.3: consultant has no linked account in this system
 		}