refactor(briefwechsel): extract TagChipList from ThumbnailRow

Lifts the three-chip-plus-"+N" tag row out of ThumbnailRow into a
standalone TagChipList component so the chip cap + overflow policy
lives in one place and can be reused on other surfaces (document
detail header is a candidate). ThumbnailRow drops from 110 to ~90
lines and no longer owns tag-slicing logic — it just asks for the
list with max=3.

Behavior is byte-identical: same data-testid, same max cap, same
"+N" overflow indicator. All ThumbnailRow row-level tag tests
continue to pass against the new composition.

Refs #305
Fixes @felixbrandt suggestion 1 from PR review

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

frontend/src/lib/components/ThumbnailRow.svelte

@@ -1,5 +1,6 @@
 <script lang="ts">
 import ConversationThumbnail from '$lib/components/ConversationThumbnail.svelte';
+import TagChipList from '$lib/components/TagChipList.svelte';
 import { formatDate } from '$lib/utils/date';
 import { relativeYearsDe } from '$lib/relativeTime';
 
@@ -36,8 +37,6 @@ let {
 } = $props();
 
 const title = $derived(doc.title || doc.originalFilename);
-const displayedTags = $derived((doc.tags ?? []).slice(0, 3));
-const hiddenTagCount = $derived(Math.max(0, (doc.tags ?? []).length - 3));
 const otherPartyName = $derived(
 	showOtherParty
 		? isOut
@@ -91,19 +90,6 @@ const ariaLabel = $derived(
 			{/if}
 		</div>
 
-		{#if displayedTags.length > 0}
-			<div class="flex flex-wrap items-center gap-1 pt-0.5">
-				{#each displayedTags as tag (tag.id)}
-					<span
-						data-testid="thumb-row-tag"
-						class="max-w-[140px] truncate rounded-full border border-line bg-surface px-2 py-0.5 text-xs text-ink-2"
-						>{tag.name}</span
-					>
-				{/each}
-				{#if hiddenTagCount > 0}
-					<span class="text-xs font-bold text-ink-3">+{hiddenTagCount}</span>
-				{/if}
-			</div>
-		{/if}
+		<TagChipList tags={doc.tags ?? []} max={3} />
 	</div>
 </a>