fix: language selector speech bubble, profile menu on mobile, hide redundant hamburger

- LanguageSelector: add speech bubble tail, replace green active with
  nord8 blue + dark text, remove floating gap
- Header: hide hamburger menu on mobile when no links, show profile
  picture directly in top bar instead
- UserHeader: center mobile dropdown, fix tail color/position, add
  profile picture overlay to tuck tail behind, add drop shadow
- Main layout: stop passing empty links snippet

src/routes/(main)/+layout.svelte

@@ -8,11 +8,6 @@ let user = $derived(data.session?.user);
 </script>
 
 <Header>
-	{#snippet links()}
-		<ul class=site_header>
-		</ul>
-	{/snippet}
-
 	{#snippet language_selector_mobile()}
 		<LanguageSelector />
 	{/snippet}