fix: stale-while-revalidate for offline reads + axios timeout

navigator.onLine is unreliable on Android — returns true whenever any
network interface is up, regardless of actual reachability. This caused
all repo reads to take the API branch and either wait 5 s for the SW
NetworkFirst timeout (cache hit) or hang indefinitely (cache miss).

- All read repos (list/get) now return cached IndexedDB data instantly
  and carry a background refresh promise that resolves to fresh data or
  null on failure. Callers that opted in (loadTrip, loadTrips) apply
  fresh data silently when it arrives.
- tripStore.loadTrip: Promise.all now reads all 7 resources from
  IndexedDB (instant), fires network refreshes in background, sets
  isLoading: false immediately, then applies fresh data via a second
  Promise.all when ready. Tags/categories use upsertTags/upsertCategories.
- DashboardPage.loadTrips: same pattern — renders from cache instantly,
  silently updates trip list on refresh.
- axios timeout set to 8 s so requests can never hang indefinitely.
- SW networkTimeoutSeconds lowered from 5 to 2 as defence in depth.

client/src/api/client.ts

@@ -33,6 +33,7 @@ function translateRateLimit(): string {
 export const apiClient: AxiosInstance = axios.create({
   baseURL: '/api',
   withCredentials: true,
+  timeout: 8000,
   headers: {
     'Content-Type': 'application/json',
   },