feat: add Zustand stores with undo/redo and debounced persistence

- App store: theme, view routing, board list, settings
- Board store: all card/column/label/checklist/attachment mutations
- zundo temporal middleware for undo/redo (50 step limit)
- Debounced saves (500ms) with immediate flush on close

src/stores/app-store.ts

@@ -0,0 +1,69 @@
+import { create } from "zustand";
+import type { AppSettings } from "@/types/settings";
+import type { BoardMeta } from "@/types/board";
+import { loadSettings, saveSettings, listBoards, ensureDataDirs } from "@/lib/storage";
+
+export type View = { type: "board-list" } | { type: "board"; boardId: string };
+
+interface AppState {
+  settings: AppSettings;
+  boards: BoardMeta[];
+  view: View;
+  initialized: boolean;
+
+  init: () => Promise<void>;
+  setTheme: (theme: AppSettings["theme"]) => void;
+  setView: (view: View) => void;
+  refreshBoards: () => Promise<void>;
+  addRecentBoard: (boardId: string) => void;
+}
+
+function applyTheme(theme: AppSettings["theme"]): void {
+  const root = document.documentElement;
+  if (theme === "system") {
+    const prefersDark = window.matchMedia("(prefers-color-scheme: dark)").matches;
+    root.classList.toggle("dark", prefersDark);
+  } else {
+    root.classList.toggle("dark", theme === "dark");
+  }
+}
+
+export const useAppStore = create<AppState>((set, get) => ({
+  settings: { theme: "system", dataDirectory: null, recentBoardIds: [] },
+  boards: [],
+  view: { type: "board-list" },
+  initialized: false,
+
+  init: async () => {
+    await ensureDataDirs();
+    const settings = await loadSettings();
+    const boards = await listBoards();
+    set({ settings, boards, initialized: true });
+    applyTheme(settings.theme);
+  },
+
+  setTheme: (theme) => {
+    const settings = { ...get().settings, theme };
+    set({ settings });
+    saveSettings(settings);
+    applyTheme(theme);
+  },
+
+  setView: (view) => set({ view }),
+
+  refreshBoards: async () => {
+    const boards = await listBoards();
+    set({ boards });
+  },
+
+  addRecentBoard: (boardId) => {
+    const settings = get().settings;
+    const recent = [
+      boardId,
+      ...settings.recentBoardIds.filter((id) => id !== boardId),
+    ].slice(0, 10);
+    const updated = { ...settings, recentBoardIds: recent };
+    set({ settings: updated });
+    saveSettings(updated);
+  },
+}));