perf(tx-cache): skip Argon2id for unencrypted wallets

Opening the transaction-history cache always derived its encryption key
with Argon2id, even for an unencrypted wallet. For those wallets the
passphrase is just a value derivable from the database itself, so the
~50-100 ms Argon2id stall bought no additional secrecy and ran on the
UI thread at open.

- Unencrypted wallets now derive the cache key with a cheap,
  domain-separated BLAKE2b KDF over the wallet identity, keyed by the
  per-wallet salt. The secret is derivable from the DB, so this is
  security-equivalent for the unencrypted case and avoids the stall.
- Passphrase-encrypted wallets are unchanged: they keep the Argon2id
  key derivation. The two paths use separate domain labels, so the
  fast-path key and the Argon2id key never collide for the same DB and
  salt; a cache written under one cannot be read under the other.

Tests cover the fast-path round-trip and reopen, and assert that an
Argon2id unlock of the same DB produces a different key and fails to
read the fast-path snapshot.

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

src/data/transaction_history_cache.h

@@ -61,6 +61,9 @@ public:
 
     bool ensureOpen();
     bool unlockWithPassphrase(const std::string& walletIdentity, const std::string& passphrase);
+    // Unlock an UNENCRYPTED wallet's cache without Argon2id: the "secret" is derivable from the DB, so a
+    // cheap domain-separated BLAKE2b KDF is security-equivalent and avoids a ~50-100 ms UI-thread stall.
+    bool unlockUnencrypted(const std::string& walletIdentity);
     void lockKey();
     bool hasKey() const;
     bool isUnlockedFor(const std::string& walletIdentity) const;
@@ -89,6 +92,9 @@ private:
     bool exec(const char* sql);
     bool createSchema();
     std::vector<unsigned char> getOrCreateSalt(const std::string& walletHash);
+    static bool deriveKeyFast(const std::string& walletHash,
+                              const std::vector<unsigned char>& salt,
+                              std::array<unsigned char, 32>& keyOut);   // BLAKE2b, label-separated
     static bool deriveKey(const std::string& passphrase,
                           const std::vector<unsigned char>& salt,
                           std::array<unsigned char, 32>& keyOut);