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 In doing so, some parts of the codebase have become outdated, unnecessary, or overly-complex as a natural result of this feature prioritization.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/controllers/application_controller.rb

@@ -22,12 +22,6 @@ class ApplicationController < ActionController::Base
       subscribed_at.present? && subscribed_at <= Time.current && subscribed_at > 1.hour.ago
     end
 
-    def with_sidebar
-      return "turbo_rails/frame" if turbo_frame_request?
-
-      "with_sidebar"
-    end
-
     def detect_os
       user_agent = request.user_agent
       @os = case user_agent