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>

src/config/settings.cpp

@@ -201,6 +201,7 @@ bool Settings::load(const std::string& path)
         loadScalar(j, "wizard_completed", wizard_completed_);
         loadScalar(j, "seed_backup_reminded", seed_backup_reminded_);
         loadScalar(j, "daemon_update_prompted_size", daemon_update_prompted_size_);
+        loadScalar(j, "active_wallet_file", active_wallet_file_);
         loadScalar(j, "seed_migration_pending", seed_migration_pending_);
         loadScalar(j, "seed_migration_dest", seed_migration_dest_);
         loadScalar(j, "seed_migration_temp_dir", seed_migration_temp_dir_);
@@ -440,6 +441,7 @@ bool Settings::save(const std::string& path)
     j["wizard_completed"] = wizard_completed_;
     j["seed_backup_reminded"] = seed_backup_reminded_;
     j["daemon_update_prompted_size"] = daemon_update_prompted_size_;
+    j["active_wallet_file"] = active_wallet_file_;
     j["seed_migration_pending"] = seed_migration_pending_;
     j["seed_migration_dest"] = seed_migration_dest_;
     j["seed_migration_temp_dir"] = seed_migration_temp_dir_;