Fix landmark accessibility violation for progress and clock elements

[Copilot]

The .progress and .clock elements were not contained by landmark regions, violating WCAG 2.1 and triggering axe/region failures.

Changes

• index.html: Changed <div class="progress"> and <div class="clock"> to <aside> elements with unique aria-label attributes
• scripts/test-landmarks.sh: Added regression test verifying landmark structure
• package.json: Integrated landmark test into test suite
• AGENTS.md: Updated documentation to reflect new element structure

Implementation

<!-- Before -->
<div class="progress"></div>
<div class="clock"></div>

<!-- After -->
<aside class="progress" aria-label="Presentation progress"></aside>
<aside class="clock" aria-label="Presentation timer"></aside>

The <aside> element provides the implicit complementary landmark role required by WCAG. Existing CSS and JavaScript continue to work unchanged as they reference class selectors, not element types.

Screenshot

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>Accessibility issue: All page content should be contained by landmarks on /whisper-slides/</issue_title>
<issue_description>## What
An accessibility scan flagged the element <div class="clock" aria-label="clock"> on https://mgifford.github.io/whisper-slides/ because all page content should be contained by landmarks. Learn more about why this was flagged by visiting https://dequeuniversity.com/rules/axe/4.11/region?application=playwright.

To fix this, ensure all page content is contained by landmarks.

Specifically:

Fix any of the following:

• Some page content is not contained by landmarks

Acceptance Criteria

• The specific axe violation reported in this issue is no longer reproducible.
• The fix MUST meet WCAG 2.1 guidelines OR the accessibility standards specified by the repository or organization.
• A test SHOULD be added to ensure this specific axe violation does not regress.
• This PR MUST NOT introduce any new accessibility issues or regressions.

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Accessibility issue: All page content should be contained by landmarks on /whisper-slides/ #4

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