Skip remote-interval polling for accounts with an active push channel

[gnacho]

The official client uses push notifications as the source of truth and only polls ETags for folders whose account has NO working push channel. FolderMan::slotEtagPollTimerTimeout (src/gui/folderman.cpp:1131) filters:

std::copy_if(..., [](Folder *folder) {
    const auto account = folder->accountState()->account();
    return !pushNotificationsFilesReady(account);
});

So a folder with live push is never polled on the interval; polling is the fallback for accounts without push.

Current state in nextsync-rs: scheduler.rs arms RemoteInterval whenever remote_interval_enabled is on (scheduler.rs:1185-1190), with no knowledge of whether the account push channel is connected. Every interval fires a full RemoteInterval sync for every folder even when notify_push is delivering changes in real time. Wasteful: a full reconciliation each remote_interval_minutes on top of real-time push.

Proposed work:

1. Let the account runtime tell the folder schedulers whether the push channel is ready (the push client already emits state via store_push_state).
2. In request(), downgrade/skip RemoteInterval while push is ready for that account (keep RemotePush as the real-time trigger).
3. Keep RemoteInterval as the fallback when push is disabled, not offered, or disconnected (matches the official behaviour and our OpenCloud accounts, which have no push).

Related: #183 (file-id resolution) shares the goal of not re-syncing folders with no changes.

[gnacho]

Implemented and merged in #188 (commit 4150fb9).