feat(updater): show in-app progress during app update

Replace native update dialogs with an in-app modal that shows a live
download progress bar and installing state, so updates no longer feel
abrupt. Drives the plugin's downloadAndInstall progress events through a
zustand store.

src/store/useUpdater.ts

@@ -0,0 +1,39 @@
+import { create } from "zustand";
+import type { Update } from "@tauri-apps/plugin-updater";
+
+export type UpdatePhase =
+  | "idle"
+  | "checking"
+  | "available"
+  | "downloading"
+  | "installing"
+  | "uptodate"
+  | "error";
+
+interface UpdaterState {
+  phase: UpdatePhase;
+  version: string;
+  notes: string;
+  downloaded: number;
+  total: number;
+  error: string;
+  update: Update | null;
+  set: (partial: Partial<UpdaterState>) => void;
+  reset: () => void;
+}
+
+const initial = {
+  phase: "idle" as UpdatePhase,
+  version: "",
+  notes: "",
+  downloaded: 0,
+  total: 0,
+  error: "",
+  update: null as Update | null,
+};
+
+export const useUpdater = create<UpdaterState>((set) => ({
+  ...initial,
+  set: (partial) => set(partial),
+  reset: () => set(initial),
+}));