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/pool_orders/edit.html.erb

@@ -13,9 +13,9 @@
 
         <ul id="sortable">
           <% @pool.posts.each do |post| %>
-            <li class="ui-state-default" id="pool[post_ids]_<%= post.id %>">
-              <%= post_preview(post, show_deleted: true).presence || "Hidden: Post ##{post.id}" %>
-            </li>
+            <%= tag.li id: "pool[post_ids]_#{post.id}" do -%>
+              <%= post_preview(post, show_deleted: true).presence || "Hidden: Post ##{post.id}" -%>
+            <% end -%>
           <% end %>
         </ul>
       <% else %>