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 In doing so, some parts of the codebase have become outdated, unnecessary, or overly-complex as a natural result of this feature prioritization.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

test/models/period_test.rb

@@ -0,0 +1,61 @@
+require "test_helper"
+
+class PeriodTest < ActiveSupport::TestCase
+  test "raises validation error when start_date or end_date is missing" do
+    error = assert_raises(ActiveModel::ValidationError) do
+      Period.new(start_date: nil, end_date: nil)
+    end
+
+    assert_includes error.message, "Start date can't be blank"
+    assert_includes error.message, "End date can't be blank"
+  end
+
+  test "raises validation error when start_date is not before end_date" do
+    error = assert_raises(ActiveModel::ValidationError) do
+      Period.new(start_date: Date.current, end_date: Date.current - 1.day)
+    end
+
+    assert_includes error.message, "Start date must be before end date"
+  end
+
+  test "from_key returns period for valid key" do
+    period = Period.from_key("last_30_days")
+    assert_equal 30.days.ago.to_date, period.start_date
+    assert_equal Date.current, period.end_date
+  end
+
+  test "from_key with invalid key and fallback returns default period" do
+    period = Period.from_key("invalid_key", fallback: true)
+    assert_equal 30.days.ago.to_date, period.start_date
+    assert_equal Date.current, period.end_date
+  end
+
+  test "from_key with invalid key and no fallback raises error" do
+    assert_raises ArgumentError do
+      Period.from_key("invalid_key")
+    end
+  end
+
+  test "label returns correct label for known period" do
+    period = Period.from_key("last_30_days")
+    assert_equal "Last 30 Days", period.label
+  end
+
+  test "label returns Custom Period for unknown period" do
+    period = Period.new(start_date: Date.current - 15.days, end_date: Date.current)
+    assert_equal "Custom Period", period.label
+  end
+
+  test "comparison_label returns correct label for known period" do
+    period = Period.from_key("last_30_days")
+    assert_equal "vs. last month", period.comparison_label
+  end
+
+  test "comparison_label returns date range for unknown period" do
+    start_date = Date.current - 15.days
+    end_date = Date.current
+    period = Period.new(start_date: start_date, end_date: end_date)
+    expected = "#{start_date.strftime("%b %d, %Y")} to #{end_date.strftime("%b %d, %Y")}"
+    assert_equal expected, period.comparison_label
+  end
+end