New Design System + Codebase Refresh (#1823)

Since the very first 0.1.0-alpha.1 release, we've been moving quickly to add new features to the Maybe app. In doing so, some parts of the codebase have become outdated, unnecessary, or overly-complex as a natural result of this feature prioritization.

Now that "core" Maybe is complete, we're moving into a second phase of development where we'll be working hard to improve the accuracy of existing features and build additional features on top of "core". This PR is a quick overhaul of the existing codebase aimed to:

- Establish the brand new and simplified dashboard view (pictured above)
- Establish and move towards the conventions introduced in Cursor rules and project design overview #1788
- Consolidate layouts and improve the performance of layout queries
- Organize the core models of the Maybe domain (i.e. Account::Entry, Account::Transaction, etc.) and break out specific traits of each model into dedicated concerns for better readability
- Remove stale / dead code from codebase
- Remove overly complex code paths in favor of simpler ones

app/views/shared/_upgrade_notification.html.erb

@@ -1,5 +1,5 @@
 <%# locals: (upgrade:) %>
-<div class="bg-white space-y-4 text-right fixed bottom-10 right-10 p-5 border border-secondary shadow-xs rounded-md z-50 max-w-[350px]">
+<div class="bg-white space-y-4 text-right fixed bottom-10 right-10 p-5 shadow-border-xs rounded-md z-50 max-w-[350px]">
   <div>
     <p><%= link_to upgrade.to_s, upgrade.url, class: "text-sm text-blue-500 underline hover:text-blue-700", target: "_blank" %></p>
     <% if upgrade.complete? %>