feat: replace static logo with dynamic header text block in app layout

- Split header padding into individual properties (top, left, right) in base.css
- Reduce logo font size from 1.5em to 1.2em in base.css
- Hide static "Snek" logo div with inline display:none in app.html
- Add new logo div with block header_text template block in app.html
- Move chat header h2 from web.html into the new header_text block in app.html

src/snek/templates/web.html

@@ -1,10 +1,9 @@
 {% extends "app.html" %}
 
+{% block header_text %}<h2>{{ name }}</h2>{% endblock %} 
+
 {% block main %}
 <section class="chat-area" id="chat">
-    <div class="chat-header">
-        <h2>{{ name }}</h2>
-    </div>
     <div class="chat-messages">
         {% for message in messages %}
             {% autoescape false %}