perf(ui): stop backdrop effects requesting frames behind a modal

While a blur overlay (a modal dialog or the lock screen) covers the
window, the theme backdrop effects kept requesting animation frames
even though they are not visible, so the app never went idle and
re-rendered continuously.

The ambient animation predicate now drops the backdrop-effects term
when a blur overlay was active last frame. Reduce Motion and the
unfocused/hidden throttle were already handled, and the overlay's own
moving elements (the lock screen's unlock dots, a spinner) still request
frames through the progress term, so nothing on screen freezes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/main.cpp

@@ -2359,8 +2359,12 @@ int main(int argc, char* argv[])
             const bool backdropNeedsFrames = (backdrop_active || app.getGradientTexture() != 0)
                                           && !opaqueBackground;
             const bool reduceMotion = app.settings() && app.settings()->getReduceMotion();
+            // C128: suppress backdrop theme-effect frame requests while a blur overlay (a modal dialog or
+            // the lock screen) covers them -- they aren't visible, and the overlay's own moving elements
+            // request frames via RequestProgressFrame(), so this never freezes anything on screen.
             const bool effectsAnimating = !reduceMotion
-                && dragonx::ui::effects::ThemeEffects::instance().hasActiveAnimation();
+                && dragonx::ui::effects::ThemeEffects::instance().hasActiveAnimation()
+                && !dragonx::ui::material::AnyBlurOverlayActiveLastFrame();
             const bool ambient = effectsAnimating
                               || backdropNeedsFrames
                               || avatarAnimating