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.cpp

@@ -388,14 +388,14 @@ void AcrylicMaterial::setQuality(AcrylicQuality quality)
     }
 }
 
-void AcrylicMaterial::applyBlur(float radius)
+void AcrylicMaterial::applyBlur(float radius, bool ignoreMultiplier)
 {
     if (!blurBuffers_.isValid() || !blurShader_.isValid()) {
         return;
     }
-    
-    // Apply blur radius multiplier from settings
-    float scaledRadius = radius * settings_.blurRadiusMultiplier;
+
+    // Apply the user's blur-strength multiplier — unless the caller wants an absolute radius.
+    float scaledRadius = ignoreMultiplier ? radius : radius * settings_.blurRadiusMultiplier;
     
     // Skip blur entirely when multiplier is at or near zero — show sharp background
     if (scaledRadius < 0.5f) {
@@ -566,7 +566,7 @@ void AcrylicMaterial::drawRect(ImDrawList* drawList, const ImVec2& pMin, const I
     }
     
     // Apply blur to captured background
-    applyBlur(params.blurRadius);
+    applyBlur(params.blurRadius, params.absoluteBlurRadius);
     
     // Calculate UV coordinates for sampling the blur FBO texture.
     // With multi-viewport enabled, ImGui draw coordinates are in
@@ -1274,11 +1274,11 @@ void AcrylicMaterial::captureLiveFramebuffer()
     blurCacheValid_  = false;
 }
 
-void AcrylicMaterial::applyBlur(float radius)
+void AcrylicMaterial::applyBlur(float radius, bool ignoreMultiplier)
 {
     if (!dx_blurSRV_[0] || !dx_blurPS_ || !dx_context_) return;
 
-    float scaledRadius = radius * settings_.blurRadiusMultiplier;
+    float scaledRadius = ignoreMultiplier ? radius : radius * settings_.blurRadiusMultiplier;
     if (blurCacheValid_ && std::abs(scaledRadius - lastBlurRadius_) < 0.1f) return;
 
     int passes = 1;
@@ -1540,7 +1540,7 @@ void AcrylicMaterial::drawRect(ImDrawList* drawList, const ImVec2& pMin, const I
     }
 
     // Run DX11 blur pipeline
-    applyBlur(params.blurRadius);
+    applyBlur(params.blurRadius, params.absoluteBlurRadius);
 
     // DX11 UV: top-left = (0,0), bottom-right = (1,1)  — same as ImGui
     // With multi-viewport, pMin/pMax are in OS screen space; subtract
@@ -1704,7 +1704,7 @@ void AcrylicMaterial::resize(int, int) {}
 void AcrylicMaterial::captureBackground() {}
 void AcrylicMaterial::captureBackgroundDirect() {}
 void AcrylicMaterial::captureLiveFramebuffer() {}
-void AcrylicMaterial::applyBlur(float) {}
+void AcrylicMaterial::applyBlur(float, bool) {}
 ImTextureID AcrylicMaterial::getBlurredTexture() const { return 0; }
 ImTextureID AcrylicMaterial::getNoiseTexture() const { return 0; }
 void AcrylicMaterial::refreshCapabilities() {}

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

src/ui/material/draw_helpers.h

@@ -743,6 +743,7 @@ inline void DrawFullWindowBlurBackdrop(ImDrawList* dl, const ImVec2& pMin, const
         // caller — no other radius contends for the shared, radius-keyed blur cache.
         auto params = GetCurrentAcrylicTheme().card;
         params.blurRadius = 96.0f;                            // strong full-window blur (deeper than panels)
+        params.absoluteBlurRadius = true;                     // fixed modal blur — independent of the user's acrylic slider
         params.fallbackColor.w = 1.0f;                        // full-strength blur so the live content reads
         effects::ImGuiAcrylic::DrawAcrylicRect(dl, pMin, pMax, params, 0.0f);
         // Dim over the blur so the floating (card-less) modal reads as foreground. On DARK themes the