Fix counter, add input binding, syntax highlighting, dark mode

- Fix increment button: custom action handler instead of no-op setState
- Toggle visibility on Show/Hide Details button via $cond
- Input uses $bindState + useBoundProp for two-way binding
- Add shiki syntax highlighting (catppuccin-latte/mocha dual theme)
- Dark mode via prefers-color-scheme with CSS variables
- Layout: Live UI left, JSX Source + JSON Output stacked right

Co-authored-by: Ona <no-reply@ona.com>

example/specs/full.tsx

@@ -47,12 +47,7 @@ export const fullSpec = render(
     <Card title="Watched Input">
       <Input
         placeholder="Type a country..."
-        on={{
-          change: {
-            action: "setState",
-            params: { statePath: "/country", value: { $event: "/value" } },
-          },
-        }}
+        value={{ $bindState: "/country" }}
       />
       <Text
         content={{ $state: "/country" }}