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/services/wallet_scan_governor.h

@@ -78,7 +78,10 @@ ScanVisibility scanVisibilityForPage(ui::NavPage page, bool windowBackground);
 enum class BalancePollDecision { Skip, AskGovernor, Explicit };
 struct BalancePollInputs {
     bool allowBalance = true;      // false for the locked wallet's chain-state poll
-    bool syncing = false;          // the node is catching up (or settling after it)
+    bool syncing = false;          // the node is catching up (or settling after it): only a first read runs
+    bool background = false;       // the window is in the background: no first read while syncing
+    bool firstReadTried = false;   // the first read while syncing was already sent this connection
+    bool gateOpen = true;          // the connect gate (C44): closed right after a connect, at most ~20 s
     bool forced = false;           // the user's own send
     bool neverFetched = false;     // no balance yet this session
     bool changed = true;           // the governor's change gate