feat(fp): WP-09 — pure-logic closure: dates + missing command specs

Consolidate four hand-rolled nl-NL date formatters (tasks.ts, aanvraag-
block, letter-preview, aanvraag-view -- one more than the WP found) into
one shared/kernel/datum.ts::formatDatumNl, spec-pinned and empty-safe.
Add the two missing command specs CLAUDE.md's testing rule calls for:
draft-sync.spec.ts (debounce coalescing + trailing-call + submit Result
shape, via fake timers) and submit-change-request.spec.ts. Remove the
unused RemoteData.map3 (updating the three docs that mentioned it); the
variant input on confirmation.component.ts was already gone. Documents
both stale-WP-text corrections in the backlog file.

This closes out backlog Phase 1 (FP/DDD core, WP-05..09).

src/app/brief/ui/letter-preview/letter-preview.component.ts

@@ -3,6 +3,7 @@ import { NgTemplateOutlet } from '@angular/common';
 import { HeadingComponent } from '@shared/ui/heading/heading.component';
 import { ButtonComponent } from '@shared/ui/button/button.component';
 import { PlaceholderChipComponent } from '@shared/ui/placeholder-chip/placeholder-chip.component';
+import { formatDatumNl } from '@shared/kernel/datum';
 import { Paragraph } from '@shared/kernel/rich-text';
 import { Brief, LetterBlock } from '@brief/domain/brief';
 import { Diagnostic } from '@brief/domain/placeholders';
@@ -152,11 +153,7 @@ export class LetterPreviewComponent {
   hideSampleLabel = input($localize`:@@brief.preview.hideSample:Testwaarden verbergen`);
 
   protected showSample = signal(false);
-  private today = new Date().toLocaleDateString('nl-NL', {
-    day: 'numeric',
-    month: 'long',
-    year: 'numeric',
-  });
+  private today = formatDatumNl(new Date());
 
   private defs = computed(() => new Map(this.brief().placeholders.map((p) => [p.key, p])));
   private worst = computed(() => {