fix(security): dim the lock-screen backdrop so the card pops in all themes

GUI review (light theme) showed the blurred backdrop is light there, washing out
the (now glass) card. Add a theme-independent dark scrim over the blur so the lock
screen reads as a clearly-dimmed "locked" state and the card stands out in both
light and dark themes — standard lock-screen dimming. Alpha tunable.

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

src/app_security.cpp

@@ -793,6 +793,10 @@ void App::renderLockScreen() {
             cap.captureFrames--;
         }
         DrawFullWindowBlurBackdrop(dl, winPos, winMax, /*allowBlur=*/!justLocked);
+        // A lock screen should read as a clearly-dimmed "locked" state; the blur alone is light in a
+        // light theme, which washes out the card. Add a theme-independent dark scrim over the blur so
+        // the card pops in both themes (standard lock-screen dimming).
+        dl->AddRectFilled(winPos, winMax, IM_COL32(6, 8, 16, 140));
         MarkBlurOverlayDrawn(nullptr);  // suppress foreground theme-effect bleed + re-capture on unlock
     }