fix: remove navigator.onLine guards and fix upsert races in all repos

navigator.onLine returns false transiently during service worker activation
(skipWaiting + clientsClaim), causing all repo refresh IIFEs to return null
immediately on first page load — leaving the UI with empty data until F5.

Fixes applied across all list repos (trip, day, place, packing, todo, budget,
reservation, accommodation, file):
- Drop navigator.onLine guard; let fetch fail naturally when truly offline
- Await all upsert calls (some were fire-and-forget, risking race conditions
  against subsequent reads and silent swallowed failures)
- Return Promise.resolve(null) instead of Promise.resolve(fresh) in the
  IDB-empty network path, so loadTrip's background refresh Promise.all
  resolves null and skips set({trip}), preventing a spurious reference change
  that was resetting the 1500ms splash timer

Tests updated: placeRepo and packingRepo "empty cache" tests now simulate
genuine network failure (HttpResponse.error) instead of relying on the
navigator.onLine guard that no longer exists; DashboardPage tests clear IDB
before each test and use a query-safe assertion after background refresh.

client/src/repo/todoRepo.ts

@@ -11,10 +11,9 @@ export const todoRepo = {
       .toArray()
 
     const refresh = (async () => {
-      if (!navigator.onLine) return null
       try {
         const result = await todoApi.list(tripId)
-        upsertTodoItems(result.items)
+        await upsertTodoItems(result.items)
         return result
       } catch {
         return null
@@ -25,7 +24,7 @@ export const todoRepo = {
 
     const fresh = await refresh
     if (!fresh) return { items: [], refresh: Promise.resolve(null) }
-    return { items: fresh.items, refresh: Promise.resolve(fresh) }
+    return { items: fresh.items, refresh: Promise.resolve(null) }
   },
 
   async create(tripId: number | string, data: Record<string, unknown>): Promise<{ item: TodoItem }> {