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>

src/app.h

@@ -839,7 +839,6 @@ private:
 
     // Private methods - rendering
     void renderStatusBar();
-    void renderAboutDialog();
     void renderLiteFirstRunPrompt();  // lite-only welcome modal when no wallet exists yet
     void renderLiteUnlockPrompt();    // lite-only send-time unlock modal
     void renderImportKeyDialog();