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/types/template.ts

@@ -0,0 +1,15 @@
+import type { ColumnWidth, Label, BoardSettings } from "./board";
+
+export interface BoardTemplate {
+  id: string;
+  name: string;
+  color: string;
+  columns: {
+    title: string;
+    width: ColumnWidth;
+    color: string | null;
+    wipLimit: number | null;
+  }[];
+  labels: Label[];
+  settings: BoardSettings;
+}