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/brief/application/brief.store.spec.ts

@@ -301,3 +301,28 @@ describe('BriefStore.revealBigNummer (PRD-0002 §5c)', () => {
     expect(store.lastError()).toBe('geweigerd');
   });
 });
+
+describe('BriefStore.flushPending (CanDeactivate guard / beforeunload)', () => {
+  const okSave = () =>
+    vi.fn(() => Promise.resolve({ ok: true, value: filledView } as Result<string, BriefView>));
+
+  it('flushes a pending debounced edit immediately and clears the pending flag', async () => {
+    const save = okSave();
+    const store = await loadedStore({ save });
+    expect(store.hasPendingSave()).toBe(false);
+
+    store.edit({ tag: 'FreeTextBlockAdded', sectionKey: 'kern' });
+    expect(store.hasPendingSave()).toBe(true); // 600ms debounce armed, not yet fired
+
+    await store.flushPending();
+    expect(save).toHaveBeenCalledTimes(1); // no timer wait needed
+    expect(store.hasPendingSave()).toBe(false); // timer consumed
+  });
+
+  it('is a no-op when no edit is pending', async () => {
+    const save = okSave();
+    const store = await loadedStore({ save });
+    await store.flushPending();
+    expect(save).not.toHaveBeenCalled();
+  });
+});