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/util/i18n.cpp

@@ -1060,6 +1060,9 @@ void I18n::loadBuiltinEnglish()
         // --- Address Book Dialog ---
         strings_["address_book_add"] = "Add Address";
         strings_["address_book_add_new"] = "Add New";
+        strings_["contact_global"] = "Show in every wallet (global contact)";
+        strings_["contact_global_tt"] = "On: this contact stays visible no matter which wallet you load. Off: it belongs to the current wallet only.";
+        strings_["contact_global_badge_tt"] = "Global contact — visible in every wallet";
         strings_["address_book_added"] = "Address added to book";
         strings_["address_book_count"] = "%zu addresses saved";
         strings_["address_book_deleted"] = "Entry deleted";