fix(gateway): declare viewer presence explicitly instead of deriving it from subscriptions (#116001)

* fix(gateway): declare viewer presence explicitly instead of deriving it from subscriptions

* refactor(ui): isolate viewer presence lifecycle ownership

* chore(protocol): register viewer presence schema owner

* fix(ui): keep viewer presence cleared after detach

* chore: drop changelog edits from this PR

CHANGELOG.md is release-owned; release generation derives entries from merged
PRs. Release-note context stays in the PR body.

src/shared/session-types.ts

@@ -65,7 +65,7 @@ export type SessionsListResultBase<TDefaults, TRow> = {
   nextOffset?: number | null;
   hasMore?: boolean;
   /** Complete creator facet for the filtered result, independent of pagination. */
-  creators?: Array<{ id: string; label?: string }>;
+  creators?: Array<{ id: string; label?: string; avatarUrl?: string }>;
   defaults: TDefaults;
   sessions: TRow[];
 };