feat: add focus-visible rings and refactor accordion chevron

Add global :focus-visible outline style and migrate form components
from :focus box-shadow to :focus-visible outline. Refactor accordion
chevron from CSS ::after pseudo-element to explicit span element.

src/ui/css/gen.clj

@@ -63,6 +63,17 @@
   margin: 0;
   background: var(--bg-0);
   color: var(--fg-0);
+}
+
+:focus-visible {
+  outline: 2px solid var(--accent);
+  outline-offset: 2px;
+  box-shadow: none;
+}
+
+:focus:not(:focus-visible) {
+  outline: none;
+  box-shadow: none;
 }")
 
 (defn collect-component-css