Build ViewComponent class for Step Indicator

[baonguyenNava]

Overview

The step indicator is currently implemented as an ERB partial at app/views/strata/shared/_step_indicator.html.erb with no backing ViewComponent class. This means host apps must use the render partial: syntax with a locals hash, which is harder to document, test, and extend than a proper ViewComponent.

Current Usage

<%= render partial: "strata/shared/step_indicator", locals: {
  steps: [:before_you_start, :personal_info, :contact, :household, :employment, :review, :submit],
  current_step: :personal_info,
  type: :counters
} %>

Proposed API

<%= render Strata::StepIndicatorComponent.new(
  steps: [:before_you_start, :personal_info, :contact],
  current_step: :personal_info,
  type: :counters,
  translation_scope: "strata.application_forms.steps"
) %>

Acceptance Criteria

• Create Strata::StepIndicatorComponent backed by the existing partial template
• Support all current locals as constructor arguments: steps, current_step, translation_scope, large_header, header_first, type
• Retain existing segment-state logic: complete / current / future
• i18n display names pulled from translation_scope; fall back to humanized symbol
• Unit tests covering each segment state and the type: :counters variant
• Update ui-kit-components.md reference to document the component API
• Backward-compatible: retain the existing partial so host apps that already use render partial: do not break (deprecated notice in partial is acceptable)

Context

The step indicator appears on every question page of every multi-step form in the SDK. Converting it to a ViewComponent improves testability, enables slot-based extension, and aligns it with the rest of the SDK's component architecture (TaskListComponent, AccordionComponent, etc.).