fix(decnet_web/Layout): theme swap end-of-animation flash

Without fill: 'forwards' the clip-path keyframes release at
animation end and the pseudo reverts to its computed style
(no clip-path), so the old layer flashes back at full size for
a frame before View Transitions tears the pseudo-elements down.
Pinning the final keyframe with fill-forwards keeps the old
layer fully clipped through to teardown.

decnet_web/src/lib/useThemeToggle.ts

@@ -86,6 +86,11 @@ function animateSwap(next: Theme, x: number, y: number): void {
         duration: 520,
         easing: 'cubic-bezier(0.4, 0, 0.2, 1)',
         pseudoElement: '::view-transition-old(root)',
+        /* Without 'forwards' the keyframes release on completion
+         * and the pseudo reverts to its computed style (no
+         * clip-path), making the old layer flash back at full
+         * size for a frame before View Transitions tears it down. */
+        fill: 'forwards',
       },
     );
   }).catch(() => { /* user-cancelled or unsupported pseudo, ignore */ });