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 In doing so, some parts of the codebase have become outdated, unnecessary, or overly-complex as a natural result of this feature prioritization.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/layouts/shared/_notifications.html.erb

@@ -0,0 +1,16 @@
+<div class="fixed z-50 bottom-6 left-6">
+  <div id="notification-tray" class="space-y-1">
+    <%= render_flash_notifications %>
+
+    <% if Current.family&.syncing? %>
+      <%= render "shared/syncing_notice" %>
+    <% end %>
+  </div>
+</div>
+
+<%= family_notifications_stream %>
+<%= family_stream %>
+
+<% if self_hosted? && (upgrade = get_upgrade_for_notification(Current.user, Setting.upgrades_mode)) %>
+  <%= render partial: "shared/upgrade_notification", locals: { upgrade: upgrade } %>
+<% end %>