feat: pinned tickets, at hand on the home page and in search

Each person pins up to 20 tickets from the ticket's page («Fijar»). They
show as a strip under the projects on the home page and in the search
palette before typing. Private and without history, like the views of
«my work» (Mongo `pinned_issues`); tickets in the trash or out of reach
are left out, and when the list is full the unreadable ones are dropped
first.

API: GET /api/me/pins, PUT/DELETE /api/issues/{issue}/pin; GraphQL
myPins, pinIssue, unpinIssue and Issue.pinned.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/web/tracker/domain/src/lib/ports/gateways.ts

@@ -140,6 +140,10 @@ export abstract class IssuesGateway {
   ): Observable<CommandResult>;
   /** The signed-in person follows (or stops following) the ticket. */
   abstract watch(issueId: string, watching: boolean): Observable<CommandResult>;
+  /** My pinned tickets, most recently pinned first. */
+  abstract pins(): Observable<IssueSummary[]>;
+  /** Pins the ticket for me (or unpins it): private, at most 20. */
+  abstract pin(issueId: string, pinned: boolean): Observable<void>;
   /** Tickets hanging from `issueId`. */
   abstract children(
     projectId: string,