feat(wallet): per-wallet data isolation foundation (P1)

First phase of multi-wallet support: keep each wallet's data separate so loading
a different wallet no longer shows the previous one's data, and lay the metadata
groundwork for the wallet-files list.

- Address book scoped per wallet: AddressBookEntry gains a "scope" ("global" or a
  wallet-identity hash); the Contacts tab shows global + current-wallet contacts
  (App::activeWalletIdentityHash) with a "show in every wallet" toggle + globe
  badge. Legacy entries migrate to "global". Scope-aware de-dup allows the same
  address across different wallets.
- Wallet metadata index (data/wallet_index -> wallets.json): file-keyed cache of
  balance / address count / identity / size / last-opened / synced-here, since
  those can't be read off a wallet.dat without loading it. Populated on connect +
  address refresh (change-detecting upsert). Plus an active_wallet_file setting
  for the -wallet=<name> switch coming in P2.

Unit-tested (testAddressBookScope, testWalletIndex): migration, visibility filter,
scope-aware de-dup, upsert change-detection, save/reload round-trip.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CMakeLists.txt

@@ -490,6 +490,7 @@ set(APP_SOURCES
     src/rpc/connection.cpp
     src/config/settings.cpp
     src/data/address_book.cpp
+    src/data/wallet_index.cpp
     src/data/exchange_info.cpp
     src/util/logger.cpp
     src/util/async_task_manager.cpp
@@ -1062,6 +1063,8 @@ if(BUILD_TESTING)
         src/util/text_format.cpp
         src/data/wallet_state.cpp
         src/data/transaction_history_cache.cpp
+        src/data/address_book.cpp
+        src/data/wallet_index.cpp
         src/daemon/lifecycle_adapters.cpp
         src/rpc/connection.cpp
         src/config/settings.cpp