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.h

@@ -42,6 +42,7 @@
 #include "resources/provision_state.h"
 #include "util/background_sampler.h"
 #include "util/coalescing_writer.h"
+#include "util/sync_eta.h"
 #include "util/fetch_backoff.h"
 #include "util/idle_tracker.h"
 #include "util/pool_stats_service.h"
@@ -1752,6 +1753,10 @@ private:
     std::atomic<bool> mining_toggle_in_progress_{false};
     // Mining error state (main-thread only — every writer is in a MainCb or render).
     std::string mining_last_error_;
+    // Sync-ETA estimator (UI thread only). onConnected/onDisconnected may run off-thread, so they set
+    // the atomic flag and the status bar consumes it before update().
+    util::SyncRateEstimator sync_eta_;
+    std::atomic<bool> sync_eta_reset_pending_{false};
     bool mining_error_av_ = false;
     void setMiningError(std::string msg, bool av = false) { mining_last_error_ = std::move(msg); mining_error_av_ = av; }