feat(ux): robust sync ETA, persist Explorer settings, gate export-all out of lite

- Sync ETA: a new pure util::SyncRateEstimator (App-owned) replaces the
  function-local statics that went stale after a rescan/restart (producing the
  "412h" ETA). It self-resets on a height drop and on connect/disconnect/rescan/
  restart (via an atomic flag, since onConnected can run off-thread), formats
  with days, and uses the Warning() token.
- Settings: the Explorer-tab URL fields + custom-fees/fetch-prices checkboxes
  now persist on edit — they were the only controls still relying on the
  About-only Save button that a user on this tab never sees.
- Export-all-keys: hidden in the lite build (which has its own key export),
  instead of dead-ending on "Not connected to the daemon".

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app_network.cpp

@@ -741,6 +741,7 @@ void App::tryConnect()
 void App::onConnected()
 {
     maint_abort_.store(false);   // maintenance calls queued before this connect have already failed (FIFO)
+    sync_eta_reset_pending_.store(true, std::memory_order_relaxed);   // fresh connect → recompute sync ETA
     ++rpc_session_;
     if (consolidation_.abort()) ui::Notifications::instance().warning(TR("merge_run_aborted_toast"));
     state_.connected = true;
@@ -854,6 +855,7 @@ void App::onConnected()
 
 void App::onDisconnected(const std::string& reason)
 {
+    sync_eta_reset_pending_.store(true, std::memory_order_relaxed);
     // Persist what the history snapshot is still missing while the rows, the address set (the cache identity),
     // the pending-send filter and the cache key are all live: state_.clear(), pending_opids_.clear() and the
     // session reset below empty them, which would turn a later flush into a silent no-op.