feat(security): focus ring on the lock-screen input

The PIN/passphrase input uses default ImGui framing, which is subtle on the glass
card — when focused there was no clear indication the field is active. Draw an
accent (Primary) ring around the input while it's focused/being edited so it's
obvious the field is ready for typing.

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

src/app_security.cpp

@@ -955,6 +955,12 @@ void App::renderLockScreen() {
         ImGui::PopStyleVar();
         ImGui::PopItemWidth();
     }
+    // Focus indicator: a clear accent ring around the input while it's focused/being edited — the
+    // default frame is subtle on the glass card, so this shows the field is active and ready to type.
+    if (ImGui::IsItemActive() || ImGui::IsItemFocused()) {
+        dl->AddRect(ImGui::GetItemRectMin(), ImGui::GetItemRectMax(),
+                    ui::material::Primary(), 6.0f, 0, 2.0f);
+    }
     cy += 40.0f + 12.0f;
 
     // Focus the input when the lock screen first appears.