security(auth): P1 Token Refresh + sessionStorage

- 新增三個 refresh 端點（member/provider/admin），revoke 舊 token 發行新 7 天 token
- axios.js / coachAxios.js：401 interceptor 改為 refresh-then-retry，含 isRefreshing queue 防並發、isRedirecting flag 防雙重 redirect
- auth.js / coachAuth.js / AuthCallbackView.vue：localStorage 全改為 sessionStorage
- echo.js / notificationAxios.js：同步改為 sessionStorage
- notifications.js：401 時停止 polling（stopPolling），不印 error log
- 新增 TokenRefreshTest.php（11 tests / 24 assertions）：refresh 成功/失效/跨角色 403

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/plugins/echo.js

@@ -4,7 +4,7 @@ import Pusher from 'pusher-js'
 window.Pusher = Pusher
 
 function getAuthToken() {
-    return localStorage.getItem('coach_token') || localStorage.getItem('token') || null
+    return sessionStorage.getItem('coach_token') || sessionStorage.getItem('token') || null
 }
 
 const echo = new Echo({