css: refactor CSS to use Tailwind-style utility classes.

Refactor CSS to use standard Tailwind-style utility classes instead of
ad-hoc rules. This eliminates a lot of single-purpose rules for specific
UI elements and standardizes margins to be more consistent throughout
the site.

Utility classes are defined manually on an as-needed basis instead of
importing Tailwind as a whole. Naming conventions mostly follow
Tailwind's conventions, otherwise they follow Bootstrap.

* https://tailwindcss.com/docs/
* https://getbootstrap.com/docs/5.0/utilities/spacing/

app/views/static/_backtrace.html.erb

@@ -1,6 +1,6 @@
 <%# backtrace %>
 
-<ul class="backtrace">
+<ul class="backtrace font-monospace p-4 mb-4">
   <% if defined?(exception) %>
     <li><%= exception.class.to_s %> exception raised</li>
   <% end %>