perf(history): write the history snapshot on change, off the UI thread

Every history refresh, address refresh, connect and lock-state check
rewrote the encrypted tx-history snapshot on the UI thread: a JSON
document of every row, its encryption, then INSERT OR REPLACE and a
prune, an estimated 40-120 ms on a 5k-transaction wallet, even when
nothing had changed.

Now a signature of what would be stored (about 1 ms on 5k rows)
decides whether to write. Confirmations count only by the thresholds
the UI reads (conflicted, pending, 1-9, 10-99, 100+); per-address scan
heights, exact confirmations and the tip drift every block and form a
separate part.

- Routine refreshes write a content change at most every 20 s (a
  deferred one is released from the update loop) and the per-block
  drift at most every 10 min. Nothing changed means no write.
- A mining-address relabel, a passphrase change, a rescan, a
  disconnect, a wallet lock and exit write whatever is not on disk yet,
  at once.
- The UI thread only copies the rows and the key into a job; a single
  background writer (newest job wins) builds, encrypts and stores it. A
  failed write is retried.
- Exit queues the final write so it overlaps the daemon stop, then
  waits up to 3 s for the writer; a disk that hangs past that keeps the
  previous snapshot.
- A loaded snapshot counts as stored, so the first identical refresh
  doesn't rewrite it. An address refresh re-keys the snapshot only when
  the address set changed.

After a hard kill the snapshot can now be up to ~20 s behind for rows
and 10 min for scan heights, exact confirmations and the tip (it was
one refresh); the first refresh replaces it either way. Lite builds,
which never read this cache, no longer write it on Refresh or a mining
relabel, and the screenshot capture mode's demo rows never reach it.

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

src/app_security.cpp

@@ -599,7 +599,9 @@ void App::changePassphrase(const std::string& oldPass, const std::string& newPas
                 memset(change_new_pass_buf_, 0, sizeof(change_new_pass_buf_));
                 memset(change_confirm_buf_, 0, sizeof(change_confirm_buf_));
                 unlockTransactionHistoryCacheWithPassphrase(newPass);
-                storeTransactionHistoryCacheIfAvailable();
+                // New key: the old ciphertext is unreadable with it. Write now unless the unlock just did
+                // (keyId() is part of the content signature, so Now skips only if the same key is submitted).
+                storeTransactionHistoryCacheIfAvailable(data::HistoryStoreMode::Now);
                 ui::Notifications::instance().info(TR("sec_passphrase_changed_successfully"));
             } else {
                 encrypt_status_ = std::string(TR("sec_failed_prefix")) + err_msg;