fix(decnet_web/Layout): outward theme reveal, no flash either end

ANTI prefers the new theme growing outward from the click point
(visually clearer cause-and-effect than the old theme burning
away). The original outward implementation flashed at the start
because the new pseudo defaulted to its computed style (no
clip-path = fully visible) for one frame before the JS animation
registered.

Switching the animation's fill from 'forwards' to 'both' enforces
the start keyframe (circle(0) at click point) before the first
paint, in addition to pinning the end keyframe through pseudo
teardown. New layer is invisible until the animation begins,
fully visible until cleanup. No flash either end.

decnet_web/src/lib/useThemeToggle.ts

@@ -70,27 +70,25 @@ function animateSwap(next: Theme, x: number, y: number): void {
       Math.max(x, window.innerWidth - x),
       Math.max(y, window.innerHeight - y),
     );
-    /* Shrink the OLD layer (on top per index.css z-index rules)
-     * away from the click point, uncovering the NEW layer that
-     * sits behind. Going outside-in instead of inside-out avoids
-     * the one-frame flash where the default-opaque new pseudo
-     * would otherwise be visible before the clip-path registers. */
+    /* Grow the NEW layer (on top per index.css z-index rules)
+     * outward from the click point, covering the OLD layer that
+     * sits behind. fill: 'both' pins both ends of the keyframe
+     * range so the start state (clipped to a 0-radius circle)
+     * is enforced before the first paint and the end state
+     * (full-viewport circle) holds through pseudo teardown —
+     * killing the flash on either end of the animation. */
     document.documentElement.animate(
       {
         clipPath: [
-          `circle(${endRadius}px at ${x}px ${y}px)`,
           `circle(0px at ${x}px ${y}px)`,
+          `circle(${endRadius}px at ${x}px ${y}px)`,
         ],
       },
       {
         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',
+        pseudoElement: '::view-transition-new(root)',
+        fill: 'both',
       },
     );
   }).catch(() => { /* user-cancelled or unsupported pseudo, ignore */ });