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/store/conversations.go

@@ -19,6 +19,7 @@ type Conversation struct {
 	Kind           string    `bson:"kind" json:"kind"` // dm | group | project
 	CustomerID     string    `bson:"customerId,omitempty" json:"customerId,omitempty"`
 	Title          string    `bson:"title,omitempty" json:"title,omitempty"`
+	CreatorID      string    `bson:"creatorId,omitempty" json:"creatorId,omitempty"`
 	ParticipantIDs []string  `bson:"participantIds" json:"participantIds"`
 	LastMessageAt  time.Time `bson:"lastMessageAt" json:"lastMessageAt"`
 	CreatedAt      time.Time `bson:"createdAt" json:"createdAt"`
@@ -104,6 +105,28 @@ func (s *Store) ConversationByID(ctx context.Context, id string) (*Conversation,
 	return &c, nil
 }
 
+// AddParticipant adds a user to a conversation (idempotent via $addToSet).
+func (s *Store) AddParticipant(ctx context.Context, convID, userID string) error {
+	_, err := s.DB.Collection("conversations").UpdateOne(ctx,
+		bson.M{"_id": convID},
+		bson.M{"$addToSet": bson.M{"participantIds": userID}})
+	if err != nil {
+		return fmt.Errorf("add participant: %w", err)
+	}
+	return nil
+}
+
+// RemoveParticipant removes a user from a conversation.
+func (s *Store) RemoveParticipant(ctx context.Context, convID, userID string) error {
+	_, err := s.DB.Collection("conversations").UpdateOne(ctx,
+		bson.M{"_id": convID},
+		bson.M{"$pull": bson.M{"participantIds": userID}})
+	if err != nil {
+		return fmt.Errorf("remove participant: %w", err)
+	}
+	return nil
+}
+
 func (s *Store) ListConversations(ctx context.Context, userID string) ([]Conversation, error) {
 	cur, err := s.DB.Collection("conversations").Find(ctx,
 		bson.M{"participantIds": userID},