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/application/src/index.ts

@@ -1,5 +1,6 @@
 export * from './lib/notifications';
 export * from './lib/facades/projects.facade';
+export * from './lib/facades/calendar.facade';
 export * from './lib/facades/project.facade';
 export * from './lib/facades/board.facade';
 export * from './lib/facades/issue.facade';

libs/web/tracker/application/src/lib/facades/calendar.facade.ts

@@ -0,0 +1,56 @@
+import { inject, Injectable, signal } from '@angular/core';
+import { CalendarFeed, CalendarGateway } from '@task-stream/web/tracker/domain';
+import { Observable } from 'rxjs';
+
+/**
+ * My calendar subscription for presenters: the secret URL to paste into a
+ * calendar app, made, renewed or dropped here. Per person and seldom
+ * touched, so it lives in this facade alone, without a store.
+ */
+@Injectable({ providedIn: 'root' })
+export class CalendarFacade {
+  private readonly gateway = inject(CalendarGateway);
+
+  readonly feed = signal<CalendarFeed | null>(null);
+  readonly loaded = signal(false);
+  readonly busy = signal(false);
+  readonly error = signal<string | null>(null);
+
+  load(): void {
+    this.run(this.gateway.mine());
+  }
+
+  /** A new URL: the previous one, if any, stops working. */
+  create(): void {
+    this.run(this.gateway.create());
+  }
+
+  revoke(): void {
+    this.busy.set(true);
+    this.gateway.revoke().subscribe({
+      next: () => this.done(null),
+      error: () => this.failed(),
+    });
+  }
+
+  private run(call: Observable<CalendarFeed | null>): void {
+    this.busy.set(true);
+    this.error.set(null);
+    call.subscribe({
+      next: (feed) => this.done(feed),
+      error: () => this.failed(),
+    });
+  }
+
+  private done(feed: CalendarFeed | null): void {
+    this.feed.set(feed);
+    this.loaded.set(true);
+    this.busy.set(false);
+  }
+
+  private failed(): void {
+    this.loaded.set(true);
+    this.busy.set(false);
+    this.error.set('No se ha podido. Inténtalo de nuevo en un momento.');
+  }
+}