feat: Phase 4 - board templates, auto-backup, version history

- BoardTemplate type and storage CRUD (save/list/delete templates)
- createBoardFromTemplate factory function
- "Save as Template" in board card context menu
- User templates shown in NewBoardDialog with delete option
- Auto-backup on save with 5-minute throttle, 10 backup retention
- VersionHistoryDialog with backup list and restore confirmation
- Version History accessible from board settings dropdown

src/components/layout/TopBar.tsx

@@ -11,13 +11,16 @@ import {
 import {
   DropdownMenu,
   DropdownMenuContent,
+  DropdownMenuItem,
   DropdownMenuRadioGroup,
   DropdownMenuRadioItem,
+  DropdownMenuSeparator,
   DropdownMenuSub,
   DropdownMenuSubContent,
   DropdownMenuSubTrigger,
   DropdownMenuTrigger,
 } from "@/components/ui/dropdown-menu";
+import { VersionHistoryDialog } from "@/components/board/VersionHistoryDialog";
 import { useAppStore } from "@/stores/app-store";
 import { useBoardStore } from "@/stores/board-store";
 import { WindowControls } from "@/components/layout/WindowControls";
@@ -32,6 +35,7 @@ export function TopBar() {
 
   const isBoardView = view.type === "board";
 
+  const [showVersionHistory, setShowVersionHistory] = useState(false);
   const [editing, setEditing] = useState(false);
   const [editValue, setEditValue] = useState("");
   const inputRef = useRef<HTMLInputElement>(null);
@@ -240,6 +244,10 @@ export function TopBar() {
                   </DropdownMenuRadioGroup>
                 </DropdownMenuSubContent>
               </DropdownMenuSub>
+              <DropdownMenuSeparator />
+              <DropdownMenuItem onClick={() => setShowVersionHistory(true)}>
+                Version History
+              </DropdownMenuItem>
             </DropdownMenuContent>
           </DropdownMenu>
         )}
@@ -279,6 +287,12 @@ export function TopBar() {
         </Tooltip>
         <WindowControls />
       </div>
+      {isBoardView && (
+        <VersionHistoryDialog
+          open={showVersionHistory}
+          onOpenChange={setShowVersionHistory}
+        />
+      )}
     </header>
   );
 }