fix(#248): address PR review concerns — i18n, aria-label, stable keys, test selectors

- Add filter_operator_and/or/and_label/or_label i18n keys to de/en/es locale files
- Add aria-label and aria-pressed to AND/OR toggle buttons in SearchFilterBar
- Add data-testid="operator-and/or" for unambiguous test targeting (fixes substring match on German "Schlagwort")
- Use stable keys (tag.id ?? tag.name) for TagInput chip and suggestion lists
- Remove aria-level from role="option" items in TagInput (invalid attribute for that role)
- Add aria-live="polite" role="status" to TagMergeZone step indicator

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

frontend/src/routes/admin/tags/[id]/TagMergeZone.svelte

@@ -50,8 +50,8 @@ const targetTag = $derived(allTags.find((t) => t.id === targetId));
 	</h3>
 	<p class="mb-4 text-xs text-ink-3">{m.admin_tag_merge_description()}</p>
 
-	<!-- Step indicator -->
-	<p class="mb-3 text-xs font-medium text-ink-3">
+	<!-- Step indicator (aria-live announces step changes to screen reader users) -->
+	<p class="mb-3 text-xs font-medium text-ink-3" aria-live="polite" role="status">
 		{step === 1 ? m.admin_tag_merge_step1() : m.admin_tag_merge_step2()}
 	</p>