fix(resilience,perf): gate send on a known tip, surface dropped/stale txs, advance cached confs, memoize explorer

Backlog mediums (display/gating/caching only — no send/sweep/encrypt call or args changed):

Resilience:
- Gate Send/Review on a KNOWN peer-derived tip (SyncInfo::sendTipReady): a fresh reconnect or peerless node
  has tip_known=false, so its local height isn't a trustworthy network tip — sending is disabled with a
  clear tooltip until it is. Lite is exempt (its own sync gate). The z_sendmany call is untouched.
- History now shows "Dropped" for a conflicted tx (confirmations < 0) and "Expired?" for one unconfirmed
  > 1h, instead of "Pending" forever (new TransactionInfo::isConflicted/isStalePending; the tab inlines the
  check on its DisplayTx fields).
- A cached z_viewtransaction row's confirmations now advance by the blocks mined since it was sampled
  (observed_height), instead of freezing at the cached value; never goes backwards; pending (0) isn't
  extrapolated.

Performance:
- Explorer block list uses loadRangeCached() (memoized on the cache revision) instead of loadRange(), so it
  only hits SQLite after the cache changes, not every frame.

Tests: testResilienceMediums (tip gate, confirmations-advance, dropped/stale predicates) + loadRangeCached
memoization in testExplorerBlockCache (isolated on its own cache). Held: Overview/Balance per-frame list
cache, peers clippers, off-thread tx-history cache load (larger refactors).

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

src/ui/windows/transactions_tab.cpp

@@ -855,7 +855,12 @@ void RenderTransactionsTab(App* app)
                     {
                         const char* statusStr;
                         ImU32 statusCol;
-                        if (tx.confirmations == 0) {
+                        const int64_t nowSec = static_cast<int64_t>(std::time(nullptr));
+                        if (tx.confirmations < 0) {   // gettransaction < 0: conflicted / double-spent / reorged out
+                            statusStr = TR("tx_dropped"); statusCol = Error();
+                        } else if (tx.confirmations == 0 && tx.timestamp > 0 && nowSec - tx.timestamp > 3600) {
+                            statusStr = TR("tx_stale_pending"); statusCol = Warning();
+                        } else if (tx.confirmations == 0) {
                             statusStr = TR("pending"); statusCol = Warning();
                         } else if (tx.confirmations < 10) {
                             snprintf(buf, sizeof(buf), TR("conf_count"), tx.confirmations);