fix(a11y): Make recurrence modal fully keyboard-accessible

[jensens]

Summary

Fixes plone/Products.CMFPlone#4272

The recurrence modal had two keyboard accessibility issues:

1. "End recurrence" only accessible when tabbing backwards — the modal's focus trap built a static list of focusable elements at open time, missing dynamically shown/hidden fields
2. "Selected dates" and "exclude" buttons unreachable without mouse — occurrence action links loaded via AJAX were never in the focus trap, and used <a href="#"> instead of proper <button> elements

Changes

• Dynamic focus trap (pat/modal/modal.js): activateFocusTrap() now re-queries visible focusable elements on each Tab keypress instead of using a static list captured at init. This benefits all modals across Plone, not just recurrence.
• Semantic buttons (occurrence.xml, display.xml, recurrence.js): Changed occurrence action links and display widget "Add"/"Delete" links from <a href="#"> to <button type="button"> for proper semantics and native keyboard support (Enter + Space).
• CSS updates (recurrence.scss): Updated selector from div.ridisplay .rimain a to div.ridisplay .rimain button. Added :focus-visible outline for occurrence action buttons.
• Tests (recurrence.test.js): Added tests verifying <button> elements are used for display controls and dynamically added date actions.

Test plan

• All 194 existing tests pass (28 suites)
• 2 new tests added and passing
• Manual testing: open an Event edit form, click "Edit" on recurrence, verify Tab reaches all controls including "End recurrence" range options and occurrence exclude/include buttons
• Verify Shift+Tab wraps correctly through the modal
• Verify the "Add"/"Delete rules" buttons in the display widget are keyboard-accessible