feat(ui): handle session_expired_{idle,absolute} in axios interceptor

Seventh commit in the session-expiration-policy series. Wires the
backend taxonomy from commit 2 through to the frontend so users see
the right page (calm banner vs plain logout) when the refresh path
fails for different reasons.

- types/auth.ts: Token gains idle_expires_at + absolute_expires_at
  (Optional ISO 8601 strings). The next commit adds the
  useAuthSessionExpiry hook that reads these.
- api/auth.ts: OAuthCallbackResponse mirrors the same two fields.
- api/client.ts: refresh-failure handler now branches on the response
  detail. session_expired_idle and session_expired_absolute both
  redirect to /login?reason=session_expired (commit 8 adds the
  banner that reads the query param); any other detail (most
  commonly invalid_refresh_token) goes to plain /login. The bare
  redirect is guarded against re-firing when the user is already on
  /login. The refresh-success path now forwards the two new fields
  into setTokens so the store stays current as the session ages.
- pages/OAuthCallbackPage.tsx: setTokens({...}) spreads
  idle_expires_at + absolute_expires_at from the OAuth response.

No new tests — authStore.test still 2/2, tsc clean. The
useAuthSessionExpiry hook and the SessionExpiryToast that consume
the new fields land in commit 8 alongside the AccountSecurity page.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/api/auth.ts

@@ -6,6 +6,8 @@ export interface OAuthCallbackResponse {
   refresh_token: string
   token_type: string
   is_new_user: boolean
+  idle_expires_at?: string | null
+  absolute_expires_at?: string | null
 }
 
 export const authApi = {

frontend/src/api/client.ts

@@ -136,15 +136,18 @@ apiClient.interceptors.response.use(
         },
       })
 
-      const { access_token, refresh_token } = response.data
+      const { access_token, refresh_token, idle_expires_at, absolute_expires_at } = response.data
       localStorage.setItem('access_token', access_token)
       localStorage.setItem('refresh_token', refresh_token)
 
-      // Sync Zustand auth store
+      // Sync Zustand auth store — include the new expiry fields so
+      // useAuthSessionExpiry stays accurate after each refresh.
       useAuthStore.getState().setTokens({
         access_token,
         refresh_token,
         token_type: 'bearer',
+        idle_expires_at,
+        absolute_expires_at,
       })
 
       isRefreshing = false
@@ -159,11 +162,28 @@ apiClient.interceptors.response.use(
       isRefreshing = false
       onRefreshFailed(refreshError)
 
-      // Refresh failed — clear tokens and redirect to login
+      // Refresh failed — clear tokens and redirect to login. The redirect
+      // target depends on WHY the refresh failed (plan §4.10):
+      // - session_expired_idle / session_expired_absolute: the user hit a
+      //   policy boundary. Show the calm "session ended for security"
+      //   banner via ?reason=session_expired.
+      // - invalid_refresh_token (or anything else): plain logout, no
+      //   banner — the user wasn't kicked by policy, the token just
+      //   wasn't recognized.
+      const refreshAxiosErr = refreshError as AxiosError
+      const refreshDetail = (refreshAxiosErr.response?.data as { detail?: string })?.detail
+      const isPolicyExpiry =
+        refreshDetail === 'session_expired_idle' ||
+        refreshDetail === 'session_expired_absolute'
+
       localStorage.removeItem('access_token')
       localStorage.removeItem('refresh_token')
       useAuthStore.getState().logout()
-      window.location.href = '/login'
+      if (!window.location.pathname.startsWith('/login')) {
+        window.location.href = isPolicyExpiry
+          ? '/login?reason=session_expired'
+          : '/login'
+      }
       return Promise.reject(refreshError)
     }
   }

frontend/src/pages/OAuthCallbackPage.tsx

@@ -103,6 +103,8 @@ export function OAuthCallbackPage() {
           access_token: result.access_token,
           refresh_token: result.refresh_token,
           token_type: result.token_type || 'bearer',
+          idle_expires_at: result.idle_expires_at,
+          absolute_expires_at: result.absolute_expires_at,
         })
         // Hydrate user / account / subscription.
         await fetchUser()

frontend/src/types/auth.ts

@@ -3,6 +3,12 @@ export interface Token {
   refresh_token: string
   token_type: string
   must_change_password?: boolean
+  // ISO 8601 UTC strings derived from the refresh JWT's idle and absolute
+  // session windows. Used by useAuthSessionExpiry to drive the
+  // "your session ends soon" toast and the forced-logout fallback when
+  // /auth/refresh rejects with session_expired_{idle,absolute}.
+  idle_expires_at?: string | null
+  absolute_expires_at?: string | null
 }
 
 export interface AuthState {