Apply review feedback: fix initialFocusRef, positioning, and mobile viewport handling

[Copilot]

Addresses technical debt from PR #7510 review comments: corrects focus management, CSS positioning, and mobile Safari viewport calculations.

Changelog

Changed

• Overlay.module.css and ActionMenu.module.css: Replace 100vh with 100dvh in max-height calculations to prevent clipping behind mobile browser chrome
• Overlay.features.stories.tsx: Attach confirmButtonRef to close button for proper initial focus behavior
• Overlay.features.stories.module.css: Add position: relative to .ScrollableContent to establish containing block for absolutely positioned close button

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; if selected, include a brief description as to why

Testing & Reviewing

Storybook "SettingMaxHeight" example validates focus trap and scrollable content behavior. Mobile Safari testing confirms overlays no longer clip behind browser UI.

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github (Learn more about how to run integration tests)

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Latest commit: 1ae6d5b

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[github-actions]

👋 Hi, this pull request contains changes to the source code that github/github-ui depends on. If you are GitHub staff, test these changes with github/github-ui using the integration workflow. Or, apply the integration-tests: skipped manually label to skip these checks.