feat: add localStorage persistence to focus store

Wraps the focus store with zustand/middleware persist to maintain
queue state across page refreshes and app restarts.

Persists:
- current: The currently focused item
- queue: Remaining items in the work queue

Not persisted (transient state):
- isLoading: Reset on mount
- error: Reset on mount

Storage key: "mc-focus-store"

This prevents losing your place in the queue when the app restarts
or the page refreshes during development.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/stores/focus-store.ts

@@ -6,6 +6,7 @@
  */
 
 import { create } from "zustand";
+import { persist } from "zustand/middleware";
 import type { FocusAction, FocusItem } from "@/lib/types";
 
 export interface FocusState {
@@ -34,7 +35,9 @@ export interface FocusState {
   setError: (error: string | null) => void;
 }
 
-export const useFocusStore = create<FocusState>((set, get) => ({
+export const useFocusStore = create<FocusState>()(
+  persist(
+    (set, get) => ({
       current: null,
       queue: [],
       isLoading: false,
@@ -102,4 +105,13 @@ export const useFocusStore = create<FocusState>((set, get) => ({
 
       setLoading: (loading) => set({ isLoading: loading }),
       setError: (error) => set({ error }),
-}));
+    }),
+    {
+      name: "mc-focus-store",
+      partialize: (state) => ({
+        current: state.current,
+        queue: state.queue,
+      }),
+    }
+  )
+);