feat: add Color Contrast Checker tool

Summary

Add a contrast-checker.html tool that checks foreground/background color pairs against WCAG 2.1 accessibility standards and suggests fixes when they fail.

Motivation

"WCAG contrast checker" and "color contrast checker" are high-volume accessibility searches. Every team doing accessibility audits needs this. It pairs naturally with the Color Picker tool (issue #122) — and unlike generic contrast checkers, this one can suggest the minimum lightness delta needed to reach a passing ratio, removing the guesswork.

Proposed UX

• Two color pickers (foreground + background) using .color-btn from the design system; accepts HEX / RGB / HSL paste
• Swap button to flip the pair
• Live contrast ratio display: e.g. 4.72 : 1
• WCAG 2.1 result badges:
  • Normal text AA (≥ 4.5 : 1) ✓ / ✗
  • Normal text AAA (≥ 7 : 1) ✓ / ✗
  • Large text AA (≥ 3 : 1) ✓ / ✗
  • Large text AAA (≥ 4.5 : 1) ✓ / ✗
  • UI components / graphical objects AA (≥ 3 : 1) ✓ / ✗
• Live preview block: sample heading, body paragraph, and button rendered in the chosen colors so you see the actual visual result
• Fix suggestion: if failing, show the adjusted foreground and background that each just barely reach AA (minimum luminance shift), as a copy-ready HEX pair
• Optional: APCA (Advanced Perceptual Contrast Algorithm) score alongside WCAG

Implementation notes

• Category: Ornaments (sits with Color Picker, Gradients)
• Slug: contrast-checker
• Icon: 🔲
• No npm dependencies — relative luminance and contrast ratio formulas are pure math (WCAG spec §1.4.3), APCA algorithm is ~40 lines inline
• All logic inline in <script>
• Follow src/_tool-template.html and run bun run generate:sections, bun run generate:index, bun run generate:favicon after adding to tools.json

Start with src/_tool-template.html and the existing Color Picker tool, then inspect tools.json and the generator commands named in the issue. Add contrast-checker with the specified color inputs, WCAG result badges, live preview, and fix suggestions, then run bun run generate:sections, bun run generate:index, and bun run generate:favicon to verify the generated project output.