refactor(data): make AddressBook an App-owned shared instance

Phase 0a of the Contacts/Chat plan. The address book was a file-scoped
singleton (`s_address_book` + `getAddressBook()`) trapped inside
address_book_dialog.cpp, reachable only from that TU. Promote it to an
App-owned member so the upcoming Contacts tab, the Send contact picker,
and a future Chat roster all read one source of truth.

- app.h: add `data::AddressBook address_book_;` + `App::addressBook()`
  accessors, next to the other owned data models.
- app.cpp: load it once in App::init() (idempotent; missing file is fine;
  purely local, no daemon dependency).
- address_book_dialog.cpp: delete the singleton + getAddressBook(); read
  through the App* the dialog already carries (dropping the dead
  `(void)app;`). show() is static so it can't reach the App — drop its
  per-open reload (the book is authoritative for the app lifetime and
  self-saves on mutation). Drop the now-unused <memory> include.

No behavior change; groundwork for the Contacts tab.

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

src/app.h

@@ -14,6 +14,7 @@
 #include <unordered_set>
 #include <nlohmann/json_fwd.hpp>
 #include "data/transaction_history_cache.h"
+#include "data/address_book.h"
 #include "data/wallet_state.h"
 #include "rpc/connection.h"
 #include "services/network_refresh_service.h"
@@ -175,6 +176,10 @@ public:
     WalletState& state() { return state_; }
     const WalletState& state() const { return state_; }
     const WalletState& getWalletState() const { return state_; }
+    // Shared contact store (Contacts tab / Send picker / future Chat roster). App-owned so
+    // every surface reads one source of truth instead of a per-dialog singleton.
+    data::AddressBook& addressBook() { return address_book_; }
+    const data::AddressBook& addressBook() const { return address_book_; }
 
     // Connection state (convenience wrappers)
     bool isConnected() const { return state_.connected; }
@@ -814,6 +819,7 @@ private:
     // PIN vault
     std::unique_ptr<util::SecureVault> vault_;
     data::TransactionHistoryCache transaction_history_cache_;
+    data::AddressBook address_book_;   // shared contact store; loaded once in init(), self-saves on mutation
     std::string pending_transaction_history_cache_passphrase_;
     bool transaction_history_cache_loaded_ = false;