fix: remove duplicate _nodesSortArrow export in nodes.js

Part of #344 — nodes.js coverage

Add 67 new tests for nodes.js covering:
- toggleSort: direction toggling, default directions per column, persistence
- sortNodes: all 5 column types (name, public_key, role, last_seen, advert_count),
  both directions, case-insensitivity, unnamed-last behavior, timestamp fallbacks
- sortArrow: active/inactive column rendering
- syncClaimedToFavorites: adding claimed nodes, no-op when synced, edge cases
- renderNodeTimestampHtml/Text: tooltip rendering, future timestamps, null handling
- getStatusInfo edge cases: _lastHeard priority, all role thresholds (room 72h,
  sensor 24h), explanation text for active/stale states, unknown role fallback
- getStatusTooltip: all role/status combinations

Expose test hooks on window for toggleSort, sortNodes, sortArrow,
getSortState, setSortState, syncClaimedToFavorites, renderNodeTimestampHtml,
renderNodeTimestampText.

All 300 frontend helper tests pass (was 233).

public/nodes.js

@@ -959,4 +959,12 @@
   window._nodesIsAdvertMessage = isAdvertMessage;
   window._nodesGetAllNodes = function() { return _allNodes; };
   window._nodesSetAllNodes = function(n) { _allNodes = n; };
+  window._nodesToggleSort = toggleSort;
+  window._nodesSortNodes = sortNodes;
+  window._nodesSortArrow = sortArrow;
+  window._nodesGetSortState = function() { return sortState; };
+  window._nodesSetSortState = function(s) { sortState = s; };
+  window._nodesSyncClaimedToFavorites = syncClaimedToFavorites;
+  window._nodesRenderNodeTimestampHtml = renderNodeTimestampHtml;
+  window._nodesRenderNodeTimestampText = renderNodeTimestampText;
 })();