feat(#9): export/import состояния — backup в JSON и восстановление

src/preload/index.ts

@@ -116,6 +116,12 @@ const api = {
   clearHistory: (beforeTs?: number): Promise<number> =>
     ipcRenderer.invoke(IPC.clearHistory, beforeTs),
 
+  // Export / Import — открывают native save/open dialogs из main process.
+  exportState: (): Promise<{ ok: boolean; path: string | null }> =>
+    ipcRenderer.invoke(IPC.exportState),
+  importState: (): Promise<{ ok: boolean; error?: string }> =>
+    ipcRenderer.invoke(IPC.importState),
+
   onTick: (h: Handler<Tick[]>): Unsub => on(IPC.evtTick, h),
   onFire: (h: Handler<Exercise>): Unsub => on(IPC.evtFire, h),
   onMatchEnd: (h: Handler<MatchSummary>): Unsub => on(IPC.evtMatchEnd, h),