fix(registry): work around CDP subpixel capture artifact

The renderer's Page.captureScreenshot can introduce a 1-2px offset
at viewport boundaries due to compositor subpixel rounding. Grid rows
are now 341px (3×341 + 2×30 = 1083), overflowing the 1080 viewport
by 3px. The overflow is clipped by overflow: hidden, but ensures any
capture offset still sees grid content rather than the body background.

Also removed flex centering from .demo-canvas — unnecessary now that
the grid fills the viewport, and it was a source of non-deterministic
positioning under multi-worker rendering.

Co-authored-by: Kanyini <onebenson@gmail.com>

registry/components/parallax-unzoom/demo.html

@@ -32,15 +32,12 @@
         height: 1080px;
         position: relative;
         overflow: hidden;
-        display: flex;
-        align-items: center;
-        justify-content: center;
       }
 
       .parallax-unzoom-grid {
         display: grid;
         grid-template-columns: repeat(5, 360px);
-        grid-template-rows: repeat(3, 340px);
+        grid-template-rows: repeat(3, 341px);
         gap: 30px;
       }
 

registry/components/parallax-zoom/demo.html

@@ -32,15 +32,12 @@
         height: 1080px;
         position: relative;
         overflow: hidden;
-        display: flex;
-        align-items: center;
-        justify-content: center;
       }
 
       .parallax-zoom-grid {
         display: grid;
         grid-template-columns: repeat(5, 360px);
-        grid-template-rows: repeat(3, 340px);
+        grid-template-rows: repeat(3, 341px);
         gap: 30px;
       }