feat: remember zoom and pan position per board across sessions

Viewport transform (zoom level + pan offset) is saved to localStorage
keyed by board ID. Restored automatically when reopening the same board.
Debounced 300ms save on zoom/pan to avoid excessive writes.

frontend/src/pages/Editor.tsx

@@ -735,6 +735,7 @@ export default function Editor({ isPublicView }: EditorProps) {
           ref={canvasRef}
           canvasState={canvasState}
           currentTool={activeTool}
+          boardId={resolvedBoardId}
           onChange={onCanvasChange}
         />
         <UserCursors