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/lib/components/TagInput.svelte

@@ -119,7 +119,7 @@ function handleKeydown(e: KeyboardEvent) {
 		class="flex min-h-[42px] flex-wrap gap-2 rounded border border-line bg-surface p-2 focus-within:border-ink focus-within:ring-1 focus-within:ring-ink"
 	>
 		<!-- Render Selected Tags -->
-		{#each tags as tag, i (i)}
+		{#each tags as tag, i (tag.id ?? tag.name)}
 			<span class="flex items-center gap-1 rounded bg-muted px-2 py-1 text-sm font-medium text-ink">
 				{#if tag.color}
 					<span
@@ -169,7 +169,7 @@ function handleKeydown(e: KeyboardEvent) {
 				<ul
 					class="absolute top-full left-0 z-50 mt-1 max-h-48 w-full overflow-y-auto rounded border border-line bg-surface shadow-lg"
 				>
-					{#each orderedSuggestions as suggestion, i (i)}
+					{#each orderedSuggestions as suggestion, i (suggestion.id ?? suggestion.name)}
 						<li
 							role="option"
 							aria-selected={i === activeIndex}