fix(storage): owner-only secret files + bound SQLite cache growth

- Write vault.dat atomically and 0600 (it holds the PIN-encrypted passphrase, so
  a world-readable copy enables an offline brute-force of the short PIN), and
  chmod the tx-history SQLite + its WAL/SHM sidecars to 0600 on open.
- The tx-history snapshot and key-salt rows are keyed on a hash of the full
  address set, which changes whenever a new address is generated — orphaning the
  prior hash's full-history blob and salt forever. pruneOtherWallets() now drops
  rows for every non-live wallet hash on each save, bounding the database.

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

src/data/transaction_history_cache.h

@@ -77,6 +77,9 @@ private:
                       std::vector<unsigned char>& nonce,
                       std::vector<unsigned char>& cipherText);
     void clearWalletByHash(const std::string& walletHash);
+    // Delete snapshot + salt rows for every wallet hash except the live one, bounding the
+    // DB so generating a new address (which changes the hash) doesn't orphan history forever.
+    void pruneOtherWallets(const std::string& keepWalletHash);
     void close();
 
     sqlite3* db_ = nullptr;