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/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