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.cpp

@@ -454,6 +454,10 @@ bool App::init()
         tryConnect();
     }
 
+    // Populate the shared contact store from disk (per-variant addressbook.json).
+    // Idempotent; a missing file is fine (returns true). Purely local — no daemon/RPC dependency.
+    address_book_.load();
+
     DEBUG_LOGF("Initialization complete\n");
     return true;
 }