Add ARIA roles to govTabs for screen reader accessibility

[cjrace]

Summary

Fixes #187

• Adds role="tablist", role="presentation", role="tab", and role="tabpanel" ARIA roles to the govTabs component, matching the GOV.UK Frontend tabs specification
• Adds aria-selected, aria-controls, and aria-labelledby attributes so screen readers correctly identify and navigate tabs
• Updates govTab.js to toggle aria-selected when switching tabs
• Adds tests verifying all ARIA roles and attributes are correctly applied

This resolves the accessibility audit finding (page 23, Sept 2025 report) where JAWS identified tabs as links rather than tabs.

Test plan

• devtools::test() — all 11 govTabs tests pass (including 4 new ARIA tests)
• devtools::check() — 0 errors, 0 warnings
• Manual test with shinyGovstyle::run_example() — verify tabs render and switch correctly
• Screen reader test — verify tabs are announced as "tab" role, not "link"

🤖 Generated with Claude Code

[cjrace]

This additional commit fixes keyboard navigation for the tabs component.

The ARIA roles added in the previous commit improved screen reader semantics, but the tabs still required keyboard users to press Tab through every tab in the list before reaching the panel content. Per the WAI-ARIA tabs pattern (which the GOV.UK Design System follows), only the active tab should be in the keyboard tab order, with arrow keys used to navigate between tabs.

Changes:

• Added tabindex management — active tab gets tabindex="0", inactive tabs get tabindex="-1", so Tab/Shift+Tab skips unselected tabs
• Added ArrowLeft/ArrowRight keydown handler to move between tabs (matching GOV.UK's own implementation)
• Extracted a shared switchTab() function used by both click and keyboard handlers
• Added test assertions for tabindex values

Note: The bundled govuk-frontend-6.1.0.min.js does include a native Tabs class with this behaviour built in, but it's exported as an ES module which doesn't integrate with Shiny's htmlDependency system. This fix stays consistent with the existing pattern of custom jQuery handlers used elsewhere in the package (e.g. accordion.js).

🤖 Generated with Claude Code