refactor(theme): remove circular reveal animation for theme switching

The View Transitions API used here crashes WebKitGTK with SIGSEGV on
Linux. Rather than gating document.startViewTransition per platform
(see PR #2502), drop the animation entirely — it's a low-value visual
flourish on a low-frequency action that doesn't justify a permanent
platform branch.

Removes the ::view-transition-* CSS block and the coordinate plumbing
in setTheme. The optional event parameter is kept on the API surface
to keep call sites compiling; they'll be cleaned up in a follow-up
commit.

src/index.css

@@ -191,41 +191,3 @@ input[type="password"]::-ms-reveal,
 input[type="password"]::-ms-clear {
   display: none;
 }
-
-::view-transition-old(root),
-::view-transition-new(root) {
-  animation: none;
-  mix-blend-mode: normal;
-}
-
-::view-transition-old(root) {
-  z-index: 1;
-}
-
-::view-transition-new(root) {
-  z-index: 9999;
-}
-
-@keyframes theme-circle-expand {
-  from {
-    clip-path: circle(
-      0% at var(--theme-transition-x, 50%) var(--theme-transition-y, 50%)
-    );
-  }
-
-  to {
-    clip-path: circle(
-      150% at var(--theme-transition-x, 50%) var(--theme-transition-y, 50%)
-    );
-  }
-}
-
-::view-transition-new(root) {
-  animation: theme-circle-expand 0.4s ease-out;
-}
-
-@media (prefers-reduced-motion: reduce) {
-  ::view-transition-new(root) {
-    animation: none;
-  }
-}