perf(net): hold wallet scans at connect, in the background and on tab entry

- Connect gate: after the node starts answering, wallet scans wait until the
  lock state is known and the node knows its tip (or the wallet has fewer
  than 500 transactions), at most 20 s. A node already behind latches the
  sync profile from the connect's getinfo. Meanwhile a list-only pass keeps
  the address list and the cached history up, and the session's first
  balance and per-address balances are read once even while catching up,
  so the wallet shows its last-synced values instead of 0.
- Background: minimized, hidden, or covered without focus (3 s debounce,
  any input cancels) polls only the chain state every 15 s. Wallet scans,
  the mining poll, price, nudges and auto-lock interaction tracking pause;
  auto-lock now fires while minimized. Leaving a held profile marks every
  timer due again.
- Tab entry marks the timers due instead of forcing scans; a stale list or
  a never-loaded history still loads on entry.
- The scan entry points own the hard gates, failed enumerations keep the
  previous lists behind a retry hold, and failed balance calls never
  overwrite known balances with 0.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/util/i18n.cpp

@@ -1119,6 +1119,8 @@ void I18n::loadBuiltinEnglish()
         strings_["no_transactions"] = "No transactions found";
         strings_["loading_transactions"] = "Loading transactions";
         strings_["tx_loading_queued"] = "Queued transaction refresh";
+        strings_["tx_loading_waits_for_sync"] = "History will update when the node finishes syncing";
+        strings_["refresh_waits_for_sync"] = "The node is still syncing. Wallet data will refresh when it finishes.";
         strings_["tx_loading_enriching_sends"] = "Checking sent transaction details (%d)";
         strings_["tx_loading_scanning_shielded"] = "Scanning shielded history (%d addresses)";
         strings_["tx_loading_refreshing_cached"] = "Refreshing wallet history (%d cached)";