fix(a11y): improve keyboard focus indicators and navigation

[Antra1705]

Description

Replaces the low-contrast :focus-visible box-shadow (12% opacity --accent-muted) with a prominent 2px offset ring using --accent and --bg CSS variables. Adds a high-contrast mode fallback, refactors NativeShareButton hardcoded blue colors to design system tokens, adds .thumb-btn:focus-visible styles to ThumbnailStrip, adds focus rings to DraggableTextOverlays, updates BaseButton with focus-visible:* Tailwind classes, and adds a KeyboardNavigation.test.tsx integration test covering Tab navigation and keyboard activation.

Related Issue

Closes #719

Type of Contribution

• Bug fix
• GSSoC contribution

Participant Info

• GitHub username: @Antra1705
• Contribution level: Advanced

Screen Recording

Screen.Recording.2026-07-14.at.10.04.45.PM.mp4

Checklist

• I have read the contribution guidelines
• My changes follow the project structure
• I have tested my changes in Chrome, Firefox, and Safari
• bun run lint passes (no ESLint errors)
• bunx tsc --noEmit passes (no TypeScript errors)
• New interactive elements have aria-label / accessible names
• No console.log statements left in
• This PR is related to a valid issue
• Screen recording attached above (required for UI/feature/design changes)

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[github-actions]

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[github-actions]

👋 Thanks for your PR, @Antra1705!

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🟠 GSSoC'26 PR detected — thanks for contributing under GirlScript Summer of Code 2026!

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Quick checklist

• PR title follows Conventional Commits (e.g. feat: add dark mode)
• Linked the issue this PR closes (e.g. Closes #123)
• Tested the changes locally (bun run dev)
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