chore(ui): remove dead dialog duplicates (Phase 2 cleanup)

Grep-verified dead, zero live callers (the live paths all go through
BeginOverlayDialog and already got the blur backdrop):
- App::renderAboutDialog (app.cpp/app.h) — dispatch uses ui::RenderAboutDialog.
- src/ui/windows/import_key_dialog.{h,cpp} (ImportKeyDialog class) — live path is
  App::renderImportKeyDialog.
- src/ui/windows/backup_wallet_dialog.{h,cpp} (BackupWalletDialog class) — live
  path is App::renderBackupDialog.
Drop the two dead files from CMakeLists. No behavior change.

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

CMakeLists.txt

@@ -474,10 +474,8 @@ set(APP_SOURCES
     src/ui/windows/shield_dialog.cpp
     src/ui/windows/request_payment_dialog.cpp
     src/ui/windows/block_info_dialog.cpp
-    src/ui/windows/import_key_dialog.cpp
     src/ui/windows/export_all_keys_dialog.cpp
     src/ui/windows/export_transactions_dialog.cpp
-    src/ui/windows/backup_wallet_dialog.cpp
     src/ui/widgets/qr_code.cpp
     src/rpc/rpc_client.cpp
     src/rpc/rpc_worker.cpp
@@ -600,10 +598,8 @@ set(APP_HEADERS
     src/ui/windows/shield_dialog.h
     src/ui/windows/request_payment_dialog.h
     src/ui/windows/block_info_dialog.h
-    src/ui/windows/import_key_dialog.h
     src/ui/windows/export_all_keys_dialog.h
     src/ui/windows/export_transactions_dialog.h
-    src/ui/windows/backup_wallet_dialog.h
     src/ui/widgets/qr_code.h
     src/rpc/rpc_client.h
     src/rpc/rpc_worker.h