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/repo/reservationRepo.ts

@@ -4,17 +4,27 @@ import { mutationQueue, generateUUID } from '../sync/mutationQueue'
 import type { Reservation } from '../types'
 
 export const reservationRepo = {
-  async list(tripId: number | string): Promise<{ reservations: Reservation[] }> {
-    if (!navigator.onLine) {
-      const cached = await offlineDb.reservations
-        .where('trip_id')
-        .equals(Number(tripId))
-        .toArray()
-      return { reservations: cached }
-    }
-    const result = await reservationsApi.list(tripId)
-    upsertReservations(result.reservations)
-    return result
+  async list(tripId: number | string): Promise<{ reservations: Reservation[]; refresh: Promise<{ reservations: Reservation[] } | null> }> {
+    const cached = await offlineDb.reservations
+      .where('trip_id')
+      .equals(Number(tripId))
+      .toArray()
+
+    const refresh = (async () => {
+      try {
+        const result = await reservationsApi.list(tripId)
+        upsertReservations(result.reservations)
+        return result
+      } catch {
+        return null
+      }
+    })()
+
+    if (cached.length > 0) return { reservations: cached, refresh }
+
+    const fresh = await refresh
+    if (!fresh) return { reservations: [], refresh: Promise.resolve(null) }
+    return { reservations: fresh.reservations, refresh: Promise.resolve(fresh) }
   },
 
   async create(tripId: number | string, data: Record<string, unknown>): Promise<{ reservation: Reservation }> {