perf(history): memoize the transaction display list instead of rebuilding it every frame

The History tab rebuilt its entire display list on every render frame: indexing all
transactions by txid, merging autoshield send+receive pairs into "shield" rows, and
std::sort-ing the result — O(N log N) plus several heap allocations at ~60fps, only to
show one 50-row page. The data is already sorted newest-first by the refresh service,
so the per-frame sort was redundant on top.

Memoize the merged+sorted list, rebuilding only when the underlying transactions
actually change. The cache key is a cheap, allocation-free FNV-1a fingerprint over the
display-relevant fields (count, last update time, and each tx's confirmations /
timestamp / type+address first char) — a new block bumps every confirmation so the key
changes and we rebuild; otherwise (the common read/scroll case) the cache is reused.
Filtering, search, and pagination still run per-frame over the cached list (cheap linear
scans that depend on interactive state).

Also document that App::shouldRefreshTransactions() is block-height/dirty driven (not
interval-gated) — the Transactions timer only paces the check; the recent-poll handles
between-block mempool/unconfirmed deltas.

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

src/app_network.cpp

@@ -716,6 +716,11 @@ void App::setCurrentPage(ui::NavPage page)
 
 bool App::shouldRefreshTransactions() const
 {
+    // NOTE: this is block-height / dirty driven, NOT interval-gated. It returns true only when a new
+    // block arrived (currentBlocks != last_tx_block_height_), the history was never fetched, or
+    // something marked it dirty (tab entry, a send, a reorg, etc.). The Transactions timer only
+    // controls how often this CHECK runs; between blocks the lightweight recent-poll
+    // (shouldRefreshRecentTransactions / TxAge) handles mempool + unconfirmed deltas instead.
     const int currentBlocks = state_.sync.blocks;
     return network_refresh_.shouldRefreshTransactions(last_tx_block_height_,
                                                         currentBlocks,