refactor(briefwechsel): ThumbnailRow captures now at prop binding

Defaults `now` in $props() destructure so each row instance freezes
its reference time at mount, instead of calling new Date() inside
the $derived every reactivity tick. No behavioural change — the
date math is stable across re-renders for a given row — but drops
the nullish-coalesce dance and is cleaner under Storybook-style
testing where a deterministic `now` is injected.

Refs #305
Fixes @felixbrandt suggestion 3 from PR review

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/components/ThumbnailRow.svelte

@@ -27,7 +27,7 @@ let {
 	doc,
 	isOut,
 	showOtherParty,
-	now
+	now = new Date()
 }: {
 	doc: Doc;
 	isOut: boolean;
@@ -46,9 +46,7 @@ const otherPartyName = $derived(
 		: ''
 );
 const relativeYearLabel = $derived(
-	doc.documentDate
-		? relativeYearsDe(new Date(doc.documentDate + 'T12:00:00'), now ?? new Date())
-		: ''
+	doc.documentDate ? relativeYearsDe(new Date(doc.documentDate + 'T12:00:00'), now) : ''
 );
 const directionLabel = $derived(isOut ? 'Gesendet' : 'Empfangen');
 const ariaLabel = $derived(