Refactor: Centralize localStorage Operations in a Storage Utility

## Problem

`AppContext.jsx` currently contains direct `localStorage` operations for:

* PAT (`oe_pat`)
* Rate limit (`oe_rate_limit`)
* Recent searches (`oe_recent`)

This couples storage logic with application state and makes it harder to maintain.

## Proposed Solution

Create a reusable storage utility at:

`src/utils/storage.js`

The utility should:

* Centralize `localStorage` access
* Define storage keys using `STORAGE_KEYS`
* Provide `get`, `set`, and `remove` methods

Then update `src/context/AppContext.jsx` to use the storage utility instead of accessing `localStorage` directly.

## Expected Result

* No direct `localStorage` calls in `AppContext.jsx`
* Storage keys have a single source of truth
* Existing PAT, rate-limit, and recent-search behaviour remains unchanged

**Type:** Code Quality Improvement

Start by reading src/context/AppContext.jsx to locate the PAT, rate-limit, and recent-search localStorage operations, then inspect the surrounding src/utils structure. Use the issue's expected results as the checklist: storage access and keys are centralized in src/utils/storage.js, AppContext.jsx has no direct localStorage calls, and existing behavior remains unchanged.

索引モデルが issue の本文から書いたものです。