fix(effects): capture the acrylic backdrop at framebuffer size on HiDPI

The acrylic effect blits its backdrop from the default framebuffer using
the logical viewport size as the source rectangle. On a HiDPI display
the framebuffer is larger than the logical size, so the blur sampled
only the lower-left corner of the screen and the glass showed the wrong
region stretched across each panel.

- The capture blit source now uses the real framebuffer pixel size
  (logical size times ImGui's DisplayFramebufferScale, truncated the
  same way the GL backend does), while the capture and blur targets stay
  at logical size. The framebuffer size is tracked separately from the
  viewport size so quality or buffer-lifetime resets cannot lose it, and
  moving the window between a Retina and a non-Retina display redoes the
  capture. DX11 is unaffected; its scale is one.
- The debug screenshot sweep captures at framebuffer size too, so HiDPI
  captures are no longer cropped.

Tests: the framebuffer-extent helper matches the GL backend's
truncation and falls back to the logical size when the scale is unknown.

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

src/ui/effects/acrylic.h

@@ -147,6 +147,13 @@ public:
      * @param height Viewport height
      */
     void resize(int width, int height);
+
+    /// Size of FB 0 in pixels (differs from the logical viewport on HiDPI GL). Used only as the capture
+    /// blit source. Kept apart from viewportWidth_/Height_ so setQuality()/syncBufferLifetime() resets
+    /// of those never lose it. DX11 ignores it (CopyResource needs identical dimensions, scale is 1).
+    void setFramebufferSize(int w, int h) {
+        if (w != fbWidth_ || h != fbHeight_) { fbWidth_ = w; fbHeight_ = h; markBackgroundDirty(); }
+    }
     
     /**
      * @brief Capture current screen content for blur source
@@ -375,6 +382,10 @@ private:
     // Screen dimensions
     int viewportWidth_ = 0;
     int viewportHeight_ = 0;
+    int fbWidth_ = 0;     // FB 0 pixel size (0 = unknown -> use the viewport size)
+    int fbHeight_ = 0;
+    int captureSrcWidth()  const { return fbWidth_  > 0 ? fbWidth_  : viewportWidth_;  }
+    int captureSrcHeight() const { return fbHeight_ > 0 ? fbHeight_ : viewportHeight_; }
     
     // Noise texture config
     int noiseTextureSize_ = 256;  // Texture resolution (larger = less repetition)