feat(modals): fixed modal-backdrop blur, independent of the acrylic slider

The modal backdrop blur was params.blurRadius (96) scaled by the user's global
acrylic blur-strength slider (blurRadiusMultiplier), so lowering the slider also
weakened every modal's backdrop. Make the modal blur a fixed hardcoded value.

Add AcrylicParams::absoluteBlurRadius: when set, applyBlur() uses the radius
as-is and skips the multiplier (threaded through both the GL and DX11 blur paths
+ the no-op stub). DrawFullWindowBlurBackdrop sets it, so the modal backdrop is
always a 96px blur regardless of the slider. Panels/other acrylic still scale
with the slider as before.

Verified: rendering a modal at blur_multiplier 0.1 vs 2.0 now produces an
identical backdrop (max pixel diff 1/255) — previously those were ~9.6px vs
~192px of blur.

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

src/ui/effects/acrylic.h

@@ -39,7 +39,12 @@ struct AcrylicParams {
     
     // Blur radius in pixels (typical: 20-60)
     float blurRadius = 30.0f;
-    
+
+    // When true, blurRadius is used AS-IS — NOT scaled by the user's blur-strength slider
+    // (blurRadiusMultiplier). The modal backdrop sets this so its blur is a fixed value, independent
+    // of the global acrylic slider.
+    bool absoluteBlurRadius = false;
+
     // Noise texture opacity (typical: 0.02-0.04)
     float noiseOpacity = 0.02f;
     
@@ -341,7 +346,7 @@ private:
     /**
      * @brief Apply blur passes to captured content
      */
-    void applyBlur(float radius);
+    void applyBlur(float radius, bool ignoreMultiplier = false);
     
     /**
      * @brief Composite final acrylic appearance