Add USWDS Button and ButtonGroup components

[MichaelCrawfordNava]

Changes

• Adds Strata::US::ButtonComponent — a USWDS-styled button rendered as either a <button> or an <a> (via href:). Supports variant:, size:, inverse:, disabled:, type:, plus the standard classes: / extra HTML attribute forwarding.
• Adds Strata::US::ButtonGroupComponent — <ul class="usa-button-group"> with each entry wrapped in <li class="usa-button-group__item">. Supports the segmented variant.
• Lookbook previews for both components.
• RSpec coverage: 31 examples for ButtonComponent, 12 for ButtonGroupComponent.
• Refactors FormBuilder#submit to delegate to ButtonComponent.css_classes and gain a :variant option for non-primary submits. The existing :big option and margin-y-6 spacing are preserved.
• Updates docs/uswds-components.md with Button and Button Group sections.

Context

Two new ViewComponents in the Strata::US namespace, following the existing one-module-per-component pattern (Alert, Breadcrumbs, Card, etc.).

ButtonComponent exposes a class method, .css_classes(variant:, size:, inverse:), that returns just the bare USWDS class string. This is intentionally the single source of truth used by:

• the component itself,
• FormBuilder#submit (this PR),
• the strata_link_to / strata_button_to view helpers (follow-up PR),
• any caller that needs to apply USWDS button styling to a Rails-rendered tag (button_to, link_to, form.button).

ButtonGroupComponent mirrors the structure of BreadcrumbsComponent — the parent renders a <ul>, each with_item slot produces a nested ItemComponent that renders as <li>.

The FormBuilder#submit refactor lives in this PR rather than the adoption PR because the component should land with at least one consumer — leaving submit on hand-written class strings would mean shipping an unused abstraction.

This PR is the base of a three-PR stack:

1. This PR — components + FormBuilder#submit refactor
2. cael/uswds-button-helpers — adds strata_link_to / strata_button_to view helpers
3. cael/uswds-button-adoption — migrates every hand-written usa-button / usa-button-group call site to the new component + helpers

Testing

$ make lint-ci
351 files inspected, no offenses detected

$ make test
1379 examples, 0 failures

Test plan:

• bundle exec rspec spec/components/strata/us/button_component_spec.rb — 31 examples cover defaults, every variant, sizes, inverse, type, href switch, disabled (button vs anchor), extra classes/HTML attributes, content escaping, and the .css_classes class method.
• bundle exec rspec spec/components/strata/us/button_group_component_spec.rb — 12 examples cover structural markup, the segmented variant, item-level classes/attributes, and content rendering/escaping.
• bundle exec rspec spec/helpers/strata/form_builder_spec.rb -e submit — regression specs covering default styling, :big, :variant, :big + :variant, and ArgumentError on unknown variants.
• Browse Lookbook (make start → http://localhost:3000/lookbook) and visually verify the Button and Button Group previews.

[Copilot]

Pull request overview

Introduces two new USWDS ViewComponents — Strata::US::ButtonComponent and Strata::US::ButtonGroupComponent — and refactors FormBuilder#submit to delegate styling to the new component's css_classes class method, which becomes the single source of truth for USWDS button class strings.

Changes:

• New ButtonComponent (button/anchor switch via href:, variant/size/inverse/disabled/type options) with a .css_classes class method for non-component call sites (e.g., button_to, link_to, f.submit).
• New ButtonGroupComponent rendering <ul class="usa-button-group"> with with_item slots and a segmented variant, following the BreadcrumbsComponent slot pattern.
• FormBuilder#submit now delegates to ButtonComponent.css_classes and accepts a :variant option; :big and margin-y-6 spacing behavior preserved. Lookbook previews, docs, and RSpec coverage added.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
app/components/strata/us/button_component.rb	New ButtonComponent class with variant/size/inverse validation and shared css_classes helper.
app/components/strata/us/button_component.html.erb	Renders <button> or <a> via content_tag.
app/components/strata/us/button_group_component.rb	New ButtonGroupComponent with nested ItemComponent slot.
app/components/strata/us/button_group_component.html.erb	Renders <ul class="usa-button-group"> with item slots.
app/previews/strata/us/button_component_preview.rb	Lookbook previews for each button variant/state.
app/previews/strata/us/button_group_component_preview.rb	Lookbook previews for default and segmented button groups.
app/helpers/strata/form_builder.rb	#submit now uses ButtonComponent.css_classes and supports :variant.
spec/components/strata/us/button_component_spec.rb	31 examples covering all options and the .css_classes helper.
spec/components/strata/us/button_group_component_spec.rb	12 examples covering structure, segmented variant, item options, and escaping.
spec/helpers/strata/form_builder_spec.rb	New regression specs for :variant, :big, combination, and unknown-variant error.
docs/uswds-components.md	Adds Button and Button Group sections plus updated ToC.

[pcunnane-nava]

This is really clean, looks great!

[lamroger-nava]

No concerns from me 👍