feat(fp): flush pending autosave before navigation/unload

Close the last-mile autosave gap: a debounced edit made in the final <600ms
before leaving a page was lost — the wizard draft-sync timer is cleared on
destroy without flushing, and root stores keep an armed timer the teardown
ignores.

New `shared/application/pending-saves.ts`: a root `PendingSaves` registry every
autosave owner joins (BriefStore, OrgTemplateStore, each createDraftSync). Two
seams flush through it — `flushPendingGuard` (CanDeactivate, on the five
autosave routes) awaits the pending write before an in-app route change; a
`beforeunload` handler (provideUnloadFlush) fires it best-effort and raises the
browser's native unsaved-changes prompt. ponytail: the HTTP seam is Angular
HttpClient (no keepalive/sendBeacon), so a hard-close flush can't be guaranteed
— hence the prompt; upgrade path noted in a comment. Each owner now nulls its
timer handle on fire so `hasPendingSave()` is accurate, and exposes
`flushPending()`.

Verified live against the running stack: navigating away 91ms after a keystroke
(well inside the debounce) fires one PUT /brief before the route changes; a
dirty reload raises the prompt, a clean reload does not. FE lint / check:tokens
/ 299 tests (+11) / build / build-storybook green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/app.config.ts

@@ -17,6 +17,7 @@ import { provideApiClient } from '@shared/infrastructure/api-client.provider';
 import { SESSION_PORT } from '@shared/application/session.port';
 import { SessionStore } from '@auth/application/session.store';
 import { provideRouteFocus } from '@shared/layout/route-focus';
+import { provideUnloadFlush } from '@shared/application/pending-saves';
 
 registerLocaleData(localeNl);
 
@@ -51,5 +52,6 @@ export const appConfig: ApplicationConfig = {
     { provide: SESSION_PORT, useExisting: SessionStore },
     { provide: LOCALE_ID, useValue: 'nl' },
     provideRouteFocus(),
+    provideUnloadFlush(),
   ],
 };