Add Contrast Checker tool

[02karthik]

What is this tool?

Contrast Checker — pick a text colour and a background colour (native swatch or hex), and instantly see the WCAG contrast ratio, per-level AA/AAA pass/fail badges, and a live preview of the pairing. It's for writers and note-takers choosing readable colours for notes, highlights, callouts, and docs — and it's a natural fit for a catalog whose own design system is built around WCAG contrast ratios.

• Ratio + verdicts: computes the WCAG 2.1 contrast ratio and shows pass/fail for Normal text (AA 4.5:1 / AAA 7:1), Large text (AA 3:1 / AAA 4.5:1), and UI & graphics (3:1).
• Live preview of the chosen pairing at heading, body, and caption sizes, plus a one-tap Swap to flip foreground/background.
• Swatch and hex fields stay in sync both directions; the last pairing persists.

Checklist

• It's a single file: contrast-checker/index.html with all CSS + JS inline.
• It is fully self-contained / offline — no remote fonts, scripts, styles, images, fetch, or import. Pure math (relative luminance), no assets.
• No analytics, trackers, or phone-home behavior.
• Added a screenshot at screenshots/contrast-checker.webp (via npm run screenshot contrast-checker).
• Registered the tool in tools.json (dir matches the folder name).
• Bumped the top-level catalogVersion (24 → 25).
• New tool — version is 1.
• npm run validate passes locally (30 tools, no warnings).

Notes for reviewers

• Design system: azure accent only, Manrope 900 display + Inter body, 24px card radius, surface-contrast elevation (no shadows). Every piece of UI chrome is token-driven, so the whole surface flips at prefers-color-scheme: dark.
• The one deliberate exception to the "never hard-code a colour" rule is the preview panel and its sample text — those are set from JS to the user's chosen colours, because they are the tool's content (the thing being tested), not chrome. This is the design system's "illustrative content" allowance. Verified both light and dark render correctly.
• Not a repeat of the closed Markdown Preview PR or the duplicate Markdown Table PRs — this is a distinct utility with no overlap with existing tools.

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