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/data/address_book.h

@@ -17,10 +17,21 @@ struct AddressBookEntry {
     std::string label;
     std::string address;
     std::string notes;
-    
+    // Per-wallet visibility scope: "global" (shown in every wallet) or a wallet-identity hash
+    // (shown only when that wallet is the active one). Empty is treated as "global" so legacy
+    // entries — written before multi-wallet scoping — keep showing everywhere.
+    std::string scope = "global";
+
     AddressBookEntry() = default;
-    AddressBookEntry(const std::string& l, const std::string& a, const std::string& n = "")
-        : label(l), address(a), notes(n) {}
+    AddressBookEntry(const std::string& l, const std::string& a, const std::string& n = "",
+                     const std::string& s = "global")
+        : label(l), address(a), notes(n), scope(s) {}
+
+    bool isGlobal() const { return scope.empty() || scope == "global"; }
+    // Visible under the wallet whose identity hash is walletHash (or globally).
+    bool visibleInWallet(const std::string& walletHash) const {
+        return isGlobal() || scope == walletHash;
+    }
 };
 
 /**
@@ -73,12 +84,21 @@ public:
     bool removeEntry(size_t index);
 
     /**
-     * @brief Find entry by address
+     * @brief Find entry by address (any scope). Used for contact-label lookups.
      * @param address Address to search for
      * @return Index or -1 if not found
      */
     int findByAddress(const std::string& address) const;
 
+    /**
+     * @brief Is there an existing entry with this address that would be visible ALONGSIDE a new
+     *        entry of the given scope? (Same wallet, or either side global.) Used to reject
+     *        duplicates within a wallet while still allowing the same address in different wallets.
+     * @param excludeIndex Storage index to skip (for edit), or -1.
+     */
+    bool hasVisibleDuplicate(const std::string& address, const std::string& scope,
+                             int excludeIndex = -1) const;
+
     /**
      * @brief Get all entries
      */