fix: resolve splash hang, dashboard skeleton, and sync-stuck regressions

- TripPlannerPage: change splash effect dep from `trip` (object ref) to
  `trip?.id` (primitive) — background refreshes no longer reset the 1500 ms
  timer on every new object reference, fixing the forever-splash on SPA nav
- tripRepo.list: await upserts on the cold-IDB path so the next mount reads
  from Dexie instead of hitting the network again, fixing the remount skeleton
- tripSyncManager: add stale-flag detection (>2 min resets _syncing), 90 s
  hard timeout via Promise.race, parallel post-sync prefetch via
  Promise.allSettled, and updated header comment to reflect manual-only policy
- OfflineTab: guard handleResync with a 120 s client-side timeout that
  interrupts and clears the spinner if syncAll stalls

client/src/repo/tripRepo.ts

@@ -36,7 +36,11 @@ export const tripRepo = {
 
     const fresh = await refresh
     if (!fresh) return { trips: [], archivedTrips: [], refresh: Promise.resolve(null) }
-    // Data came straight from network — no background re-fetch needed
+    // Await upserts on cold path so next mount reads from IDB instead of hitting network again
+    await Promise.all([
+      ...fresh.trips.map(t => upsertTrip(t)),
+      ...fresh.archivedTrips.map(t => upsertTrip(t)),
+    ]).catch(() => {})
     return { ...fresh, refresh: Promise.resolve(null) }
   },