migrate from deprecated slots to snippets and fix event handlers

- Replace deprecated <slot> syntax with modern {#snippet} and {@render} patterns
- Add TypeScript types for snippet props in Header component
- Convert on:click event handlers to onclick attribute throughout
- Update all layout files to use new snippet-based composition pattern

src/lib/components/UserHeader.svelte

@@ -136,7 +136,7 @@ h2 + p{
 </style>
 
 {#if user}
-	<button on:click={toggle_options} style="background-image: url(https://bocken.org/static/user/thumb/{user.nickname}.webp)" id=button>
+	<button onclick={toggle_options} style="background-image: url(https://bocken.org/static/user/thumb/{user.nickname}.webp)" id=button>
 	<div id=options class="speech top" hidden>
 			<h2>{user.name}</h2>
 			<p>({user.nickname})</p>