Add localStorage persistence for workbench window arrangement

Implements persistent state for instrument windows and mosaic layout across page reloads.

Changes:
- WorkbenchLayout: Save/restore activeInstruments array from localStorage
- EditorArea: Save/restore mosaic layout configuration from localStorage
- Layout is restored only if it matches currently active instruments
- Graceful error handling for localStorage failures

Fixes user request to maintain window arrangement between reloads.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: MarkoVcode

benchmesh-serial-service/frontend/src/ui/workbench/EditorArea/EditorArea.tsx

@@ -34,9 +34,30 @@ export const EditorArea: React.FC<EditorAreaProps> = ({
   React.useEffect(() => {
     if (activeInstruments.length === 0) {
       setMosaicLayout(null);
+      localStorage.removeItem('benchmesh:mosaicLayout');
       return;
     }
 
+    // Try to restore saved layout if it matches current instruments
+    try {
+      const savedLayout = localStorage.getItem('benchmesh:mosaicLayout');
+      if (savedLayout) {
+        const parsed = JSON.parse(savedLayout);
+        // Verify saved layout contains only current active instruments
+        const layoutIds = extractLayoutIds(parsed);
+        const sameInstruments =
+          layoutIds.length === activeInstruments.length &&
+          layoutIds.every((id: string) => activeInstruments.includes(id));
+
+        if (sameInstruments) {
+          setMosaicLayout(parsed);
+          return;
+        }
+      }
+    } catch (e) {
+      console.error('Failed to restore mosaic layout:', e);
+    }
+
     if (activeInstruments.length === 1) {
       setMosaicLayout(activeInstruments[0]);
       return;
@@ -66,6 +87,26 @@ export const EditorArea: React.FC<EditorAreaProps> = ({
     setMosaicLayout(buildLayout(activeInstruments));
   }, [activeInstruments]);
 
+  // Persist mosaic layout when it changes (user rearranges windows)
+  const handleMosaicChange = React.useCallback((newLayout: MosaicNode<string> | null) => {
+    setMosaicLayout(newLayout);
+    if (newLayout) {
+      try {
+        localStorage.setItem('benchmesh:mosaicLayout', JSON.stringify(newLayout));
+      } catch (e) {
+        console.error('Failed to save mosaic layout:', e);
+      }
+    }
+  }, []);
+
+  // Helper to extract all instrument IDs from a mosaic layout
+  const extractLayoutIds = (node: MosaicNode<string>): string[] => {
+    if (typeof node === 'string') {
+      return [node];
+    }
+    return [...extractLayoutIds(node.first), ...extractLayoutIds(node.second)];
+  };
+
   const renderTile = (id: string, path: MosaicBranch[]) => {
     // Parse class-specific ID format: ${instrument.id}:${classCode}
     const [instrumentId, classCode] = id.includes(':') ? id.split(':') : [id, undefined];
@@ -110,7 +151,7 @@ export const EditorArea: React.FC<EditorAreaProps> = ({
       <Mosaic<string>
         renderTile={renderTile}
         value={mosaicLayout}
-        onChange={setMosaicLayout}
+        onChange={handleMosaicChange}
         className="editor__mosaic"
       />
     </div>

benchmesh-serial-service/frontend/src/ui/workbench/WorkbenchLayout.tsx

@@ -67,7 +67,26 @@ export const WorkbenchLayout: React.FC<WorkbenchLayoutProps> = ({
   const [bottomPanelCollapsed, setBottomPanelCollapsed] = useState(true);
   const [rightPanelCollapsed, setRightPanelCollapsed] = useState(true); // Right panel not yet implemented
   const [activeView, setActiveView] = useState<ViewType>('instruments');
-  const [activeInstruments, setActiveInstruments] = useState<string[]>([]);
+
+  // Initialize activeInstruments from localStorage
+  const [activeInstruments, setActiveInstruments] = useState<string[]>(() => {
+    try {
+      const saved = localStorage.getItem('benchmesh:activeInstruments');
+      return saved ? JSON.parse(saved) : [];
+    } catch (e) {
+      console.error('Failed to load active instruments from localStorage:', e);
+      return [];
+    }
+  });
+
+  // Persist activeInstruments to localStorage whenever it changes
+  React.useEffect(() => {
+    try {
+      localStorage.setItem('benchmesh:activeInstruments', JSON.stringify(activeInstruments));
+    } catch (e) {
+      console.error('Failed to save active instruments to localStorage:', e);
+    }
+  }, [activeInstruments]);
 
   const handleInstrumentClick = (instrumentId: string) => {
     setActiveInstruments((prev) => {