feat: a calendar to subscribe to, and the backlog's corners

- Each person can subscribe their dates in Google Calendar, Outlook or
  Apple Calendar: GET /api/calendar/{token}.ics (no session) brings
  their unfinished tickets with a due date (assigned or followed), the
  planned and running sprints and the planned releases of their
  projects, as all-day events linking to the web. The token is made,
  renewed or dropped under /me/calendar (and in GraphQL); in the web,
  from «Calendario» in «Mi trabajo».
- The backlog panel keeps the selected rows' background inside its
  rounded corners.

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

libs/web/tracker/infrastructure/src/lib/adapters/graphql-calendar.gateway.ts

@@ -0,0 +1,48 @@
+import { inject, Injectable } from '@angular/core';
+import { API_CONFIG } from '@task-stream/web/shared/infrastructure';
+import { CalendarFeed, CalendarGateway } from '@task-stream/web/tracker/domain';
+import { map, Observable } from 'rxjs';
+import {
+  CreateCalendarFeedDocument,
+  MyCalendarFeedDocument,
+  RevokeCalendarFeedDocument,
+} from '../graphql/generated';
+import { GraphqlOperations } from './graphql-operations';
+
+@Injectable()
+export class GraphqlCalendarGateway extends CalendarGateway {
+  private readonly gql = inject(GraphqlOperations);
+  private readonly api = inject(API_CONFIG);
+
+  mine(): Observable<CalendarFeed | null> {
+    return this.gql
+      .query(MyCalendarFeedDocument)
+      .pipe(
+        map((d) => (d.myCalendarFeed ? this.toFeed(d.myCalendarFeed) : null)),
+      );
+  }
+
+  create(): Observable<CalendarFeed> {
+    return this.gql
+      .mutate(CreateCalendarFeedDocument, {})
+      .pipe(map((d) => this.toFeed(d.createCalendarFeed)));
+  }
+
+  revoke(): Observable<void> {
+    return this.gql
+      .mutate(RevokeCalendarFeedDocument, {})
+      .pipe(map(() => undefined));
+  }
+
+  /** The API answers the secret; calendar apps need the whole address. */
+  private toFeed(feed: { token: string; createdAt: string }): CalendarFeed {
+    const base = new URL(
+      this.api.restUrl.replace(/\/+$/, '') + '/',
+      location.origin,
+    );
+    return {
+      url: new URL(`calendar/${feed.token}.ics`, base).toString(),
+      createdAt: feed.createdAt,
+    };
+  }
+}