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/customer.go

@@ -28,9 +28,14 @@ type Ticketing struct {
 	CredentialsEnc  string        `bson:"credentialsEnc" json:"-"`
 	ProjectKey      string        `bson:"projectKey" json:"projectKey"`
 	PollIntervalSec int           `bson:"pollIntervalSec" json:"pollIntervalSec"`
-	LastSyncAt      time.Time     `bson:"lastSyncAt" json:"lastSyncAt"`
-	LastSyncStatus  string        `bson:"lastSyncStatus" json:"lastSyncStatus"` // "", ok, error
-	LastSyncError   string        `bson:"lastSyncError" json:"lastSyncError"`
+	// ConsultantIdentities maps a bounty-board consultant id to that
+	// consultant's username in THIS customer's ticketing system. It takes
+	// precedence over the consultant's global extra.ticketingIdentities so an
+	// admin can map accounts per project (§5.3).
+	ConsultantIdentities map[string]string `bson:"consultantIdentities,omitempty" json:"consultantIdentities,omitempty"`
+	LastSyncAt           time.Time         `bson:"lastSyncAt" json:"lastSyncAt"`
+	LastSyncStatus       string            `bson:"lastSyncStatus" json:"lastSyncStatus"` // "", ok, error
+	LastSyncError        string            `bson:"lastSyncError" json:"lastSyncError"`
 }
 
 // Customer == "project" in the UI (§4.2).