fix(repo): fall back to Dexie when a network read fails (H2) (#1179)

Repos gated reads on raw navigator.onLine and the online branch had no
try/catch, so a captive portal or connected-but-no-internet (navigator.onLine
lying "true") threw a network error instead of serving the good cached copy —
blanking the trip even though Dexie held it.

- new onlineThenCache(onlineFn, cacheFn) helper: reads the cache when offline,
  and on a network-level failure (Axios error with no HTTP response). A genuine
  HTTP error (4xx/5xx — the server responded) is rethrown so callers still set
  error state / navigate, not masked by a stale cache.
- gates only on navigator.onLine, NOT the connectivity probe: the probe is a
  coarse global flag and one failed health check would otherwise divert every
  read to the (possibly empty) cache even when the request would succeed.
- every repo list/get read path routed through it (reads only — writes still
  go through the mutation queue so failures surface)
- tests: captive-portal fallback, HTTP-error rethrow, non-Axios rethrow

client/src/repo/reservationRepo.ts

@@ -1,18 +1,20 @@
 import { reservationsApi } from '../api/client'
 import { offlineDb, upsertReservations } from '../db/offlineDb'
+import { onlineThenCache } from './withOfflineFallback'
 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
+    return onlineThenCache(
+      async () => {
+        const result = await reservationsApi.list(tripId)
+        upsertReservations(result.reservations)
+        return result
+      },
+      async () => ({
+        reservations: await offlineDb.reservations
+          .where('trip_id').equals(Number(tripId)).toArray(),
+      }),
+    )
   },
 }