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 In doing so, some parts of the codebase have become outdated, unnecessary, or overly-complex as a natural result of this feature prioritization.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

@@ -0,0 +1,94 @@
+class Trend
+  include ActiveModel::Validations
+
+  DIRECTIONS = %w[up down].freeze
+
+  attr_reader :current, :previous, :favorable_direction
+
+  validates :current, presence: true
+
+  def initialize(current:, previous:, favorable_direction: nil)
+    @current = current
+    @previous = previous || 0
+    @favorable_direction = (favorable_direction.presence_in(DIRECTIONS) || "up").inquiry
+
+    validate!
+  end
+
+  def direction
+    if current == previous
+      "flat"
+    elsif current > previous
+      "up"
+    else
+      "down"
+    end.inquiry
+  end
+
+  def color
+    case direction
+    when "up"
+      favorable_direction.down? ? red_hex : green_hex
+    when "down"
+      favorable_direction.down? ? green_hex : red_hex
+    else
+      gray_hex
+    end
+  end
+
+  def icon
+    if direction.flat?
+      "minus"
+    elsif direction.up?
+      "arrow-up"
+    else
+      "arrow-down"
+    end
+  end
+
+  def value
+    current - previous
+  end
+
+  def percent
+    return 0.0 if previous.zero? && current.zero?
+    return Float::INFINITY if previous.zero?
+
+    change = (current - previous).to_f
+
+    (change / previous.to_f * 100).round(1)
+  end
+
+  def percent_formatted
+    if percent.finite?
+      "#{percent.round(1)}%"
+    else
+      percent > 0 ? "＋∞" : "-∞"
+    end
+  end
+
+  def as_json
+    {
+      value: value,
+      percent: percent,
+      percent_formatted: percent_formatted,
+      current: current,
+      previous: previous,
+      color: color,
+      icon: icon
+    }
+  end
+
+  private
+    def red_hex
+      "var(--color-destructive)"
+    end
+
+    def green_hex
+      "var(--color-success)"
+    end
+
+    def gray_hex
+      "var(--color-gray)"
+    end
+end