feat(chat): passphrase-wrapped local key vault for sensitive conversations

Adds ChatSensitiveVault: a per-wallet random 32-byte key wrapped under a user
passphrase (Argon2id MODERATE KEK + XSalsa20-Poly1305) in an 89-byte local file.
This key encrypts the at-rest rows of "sensitive" conversations INDEPENDENTLY of
the seed-derived chat key, so those rows can't be read by someone who has the
seed but not this passphrase — and a wallet restored from the seed alone can't
read them at all. Losing the passphrase/file makes them unrecoverable by design.
create/unlock/changePassphrase/remove, atomic write + 0600, keyOut zeroed on any
failure. Unit-tested (round-trip, wrong-passphrase zeroing, rekey, no-clobber).

Phase B, step 2 (local vault key).

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

CMakeLists.txt

@@ -499,6 +499,7 @@ set(APP_SOURCES
     src/chat/chat_service.cpp
     src/chat/chat_database.cpp
     src/chat/chat_outgoing.cpp
+    src/chat/chat_sensitive_vault.cpp
     src/wallet/lite_owned_string.cpp
     src/wallet/lite_rollout_policy.cpp
     src/wallet/lite_client_bridge.cpp
@@ -669,6 +670,7 @@ set(APP_HEADERS
     src/chat/chat_service.h
     src/chat/chat_database.h
     src/chat/chat_outgoing.h
+    src/chat/chat_sensitive_vault.h
     src/config/version.h
     src/data/wallet_state.h
     src/data/transaction_history_cache.h
@@ -1224,6 +1226,7 @@ if(BUILD_TESTING)
         src/chat/chat_service.cpp
         src/chat/chat_database.cpp
         src/chat/chat_outgoing.cpp
+        src/chat/chat_sensitive_vault.cpp
         src/wallet/lite_owned_string.cpp
         src/wallet/lite_rollout_policy.cpp
         src/wallet/lite_client_bridge.cpp