feat(ds): DS::SegmentedControl — fix invisible dark selected pill (#2145)

* feat(ds): DS::SegmentedControl; fix invisible dark selected pill (#2137)

Ships DS::SegmentedControl — a single-select pill group (filters, mode
switches) — plus a Lookbook preview, tests, and an exemplar migration of the
budget filter tabs.

The audit flagged the dark selected pill as invisible: the bespoke controls
paired a container-inset track (gray-800) with a gray-700 active pill, barely a
step apart. The primitive uses the DS tab-token values (tab-bg-group -> a
near-black alpha-black-700 track in dark), against which the gray-700 active
pill clearly reads. Values are inlined with @variant theme-dark because
@apply-ing the custom tab utilities drops their dark override.

- app/components/DS/segmented_control.rb: slot-based with_segment(label,
  active:, href:, **opts); link or button; full_width: for equal footprint;
  selected style isolated in .segmented-control__segment--active so a controller
  can toggle it as one class.
- .segmented-control recipe in components.css.
- Migrate budgets/_budget_tabs; budget_filter_controller now toggles the single
  --active class instead of five raw utility classes.

Verified in-browser: dark active pill reads (gray-700 on near-black track);
filter toggle still works. Tests + rubocop clean.

Deferred (follow-up): auth sign-in/up switch, transaction-type tabs, and other
bespoke segmented controls — same primitive, one migration each.

* fix(a11y): expose segmented-control selection + derive active from filter param

- DS::SegmentedControl: set aria-current (links) / aria-pressed (buttons) from
  the segment's active state so screen readers announce the selection.
- budget_filter_controller: mirror aria-pressed when it toggles the active class.
- _budget_tabs: compute each segment's initial active: from params[:filter] so
  a ?filter=over_budget request server-renders the correct pill (no flash before
  Stimulus runs). Addresses CodeRabbit reviews on #2145.

* fix(ds): close unterminated segmented-control CSS rule

The --active rule swallowed the table-scroll block's comment opener and
never closed, so the Tailwind build died with 'Missing closing } at
@layer components' and both test jobs failed at boot. Restore the
closing brace and the /* opener.

Also add the budgets.show.filter.aria_label locale key the budget tabs
view referenced only through its inline default.

---------

Signed-off-by: Juan José Mata <juanjo.mata@gmail.com>
Co-authored-by: Juan José Mata <juanjo.mata@gmail.com>

test/components/previews/segmented_control_component_preview.rb

@@ -0,0 +1,20 @@
+class SegmentedControlComponentPreview < ViewComponent::Preview
+  # @display container_classes max-w-[480px]
+  # @param full_width toggle
+  def default(full_width: true)
+    render DS::SegmentedControl.new(full_width: full_width, aria_label: "Budget filter") do |sc|
+      sc.with_segment("All", active: true)
+      sc.with_segment("Over Budget")
+      sc.with_segment("On Track")
+    end
+  end
+
+  # Link segments (server-selected mode switch, e.g. the auth sign-in/up tabs).
+  # @display container_classes max-w-[320px]
+  def links
+    render DS::SegmentedControl.new(full_width: true, aria_label: "Auth mode") do |sc|
+      sc.with_segment("Sign in", href: "#", active: true)
+      sc.with_segment("Sign up", href: "#")
+    end
+  end
+end