fix: resolve WAVE accessibility issues

[saurabhhhcodes]

Summary

• removes the duplicate/nested main-content landmark by keeping the root layout as the single main landmark
• adds a programmatic label for the preset search input
• wraps text weight choices in a valid fieldset/legend group
• gives adjustment reset buttons clearer accessible names
• darkens the light-mode muted token to reduce small-text contrast alerts

Why

These changes address concrete WAVE-style accessibility findings: duplicate landmarks/IDs, unlabeled controls, invalid grouping semantics, unclear control names, and low-contrast muted text in light mode.

Closes #178

Verification

• git diff --check
• npx tsc --noEmit
• npm run lint passes with the existing react-hooks/exhaustive-deps warning in src/components/VideoEditor.tsx:276
• npm run build

Test note

• npm test -- --run is currently blocked by existing test harness issues unrelated to this patch: Vitest cannot resolve the @/utils/fontLoader alias in FFmpeg-related tests, and ThemeToggle.test.tsx runs without a localStorage shim.

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[github-actions]

👋 Thanks for your PR, @saurabhhhcodes!

Welcome to Reframe — a browser-based video editor built for everyone 🎬

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• Linked the issue this PR closes (e.g. Closes #123)
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[github-actions]

✅ PR Format Check Passed — @saurabhhhcodes

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[saurabhhhcodes]

Hi @magic-peach, this PR closes the assigned GSSoC issue #178. Could you please add/confirm the GSSoC labels (gssoc'26, gssoc:approved) during review if the scope looks good?

Note: the Vercel status is pointing to the Vercel Git authorization URL, while the repo automation checks (label, welcome, and validate) are green.