fix: set nav to full width and remove redundant float style in header

- Added `width:100%` to anchor element in base.css to ensure full-width layout
- Removed `float:right` from nav element in app.html to prevent layout overflow issues

src/snek/templates/app.html

@@ -26,7 +26,7 @@
 
   <header>
     <div class="logo no-select">{% block header_text %}{% endblock %}</div>
-    <nav class="no-select" style="float:right;overflow:hidden;scroll-behavior:smooth">
+    <nav class="no-select" style="overflow:hidden;scroll-behavior:smooth">
       <a class="no-select" href="/web.html">🏠</a>
       <a class="no-select" href="/search-user.html">🔍</a>
       <a class="no-select" style="display:none" id="install-button" href="#">📥</a>