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/messages/en.json

@@ -604,5 +604,9 @@
 	"admin_tag_delete_only_this_sub_root": "Children will become root tags",
 	"admin_tag_delete_subtree": "Delete entire subtree",
 	"admin_tag_delete_subtree_warn": "Also deletes {count} child tags",
-	"admin_tag_delete_confirm_heading": "Type «{name}» to confirm:"
+	"admin_tag_delete_confirm_heading": "Type «{name}» to confirm:",
+	"filter_operator_and": "AND",
+	"filter_operator_or": "OR",
+	"filter_operator_and_label": "All selected tags must match (AND)",
+	"filter_operator_or_label": "At least one tag must match (OR)"
 }