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.cpp

@@ -2579,41 +2579,6 @@ void App::renderLiteUnlockPrompt()
     }
 }
 
-void App::renderAboutDialog()
-{
-    auto dlg = ui::schema::UI().drawElement("inline-dialogs", "about");
-    auto dlgF = [&](const char* key, float fb) -> float {
-        auto it = dlg.extraFloats.find(key);
-        return it != dlg.extraFloats.end() ? it->second : fb;
-    };
-    
-    if (!ui::material::BeginOverlayDialog("About ObsidianDragon", &show_about_, dlgF("width", 400.0f), 0.94f)) {
-        return;
-    }
-    
-    ui::material::Type().text(ui::material::TypeStyle::H6, DRAGONX_APP_NAME);
-    ImGui::Dummy(ImVec2(0, ui::Layout::spacingSm()));
-    
-    ImGui::Text("Version: %s", DRAGONX_VERSION);
-    ImGui::Text("ImGui: %s", IMGUI_VERSION);
-    ImGui::Spacing();
-    
-    ImGui::TextWrapped("A shielded cryptocurrency wallet for DragonX (DRGX), "
-                      "built with Dear ImGui for a lightweight, portable experience.");
-    ImGui::Spacing();
-    
-    ImGui::Text("Copyright 2024-2026 The Hush Developers");
-    ImGui::Text("Released under the GPLv3 License");
-    
-    ImGui::Spacing();
-    ImGui::Separator();
-    
-    if (ui::material::StyledButton("Close", ImVec2(dlgF("close-button-width", 120.0f), 0), ui::material::resolveButtonFont((int)dlgF("button-font", 1)))) {
-        show_about_ = false;
-    }
-    
-    ui::material::EndOverlayDialog();
-}
 
 void App::renderImportKeyDialog()
 {