feat: add Color Picker / Converter tool

Summary

Add a color-picker.html tool that converts colors across HEX, RGB, HSL, and OKLCH formats with a live swatch and a mini complementary/analogous palette strip.

Motivation

Color format conversion is a daily task for designers and front-end developers. The existing Gradients tool already attracts a design audience; a dedicated Color Picker fills the natural next gap in that category. All conversion math is pure inline JS — no external library needed.

Proposed UX

• Color input: native <input type="color"> swatch + a free-text input that accepts any of the supported formats (auto-detected on paste)
• Live output cards for each format: HEX, RGB, RGBA, HSL, HSLA, OKLCH — each with a one-click copy button
• Palette strip: 5 swatches showing complementary, analogous (×2), and triadic colors derived from the input hue, each copyable
• CSS variable snippet: --color-primary: #…; and its variants ready to paste
• Clear / random color button

Implementation notes

• Category: Ornaments (sits alongside Gradients, Blob, Waves)
• Slug: color-picker
• Icon: 🎨
• No npm dependencies — all color math (HEX↔RGB↔HSL↔OKLCH) inlined; OKLCH conversion uses the standard formulas via oklch() CSS function for preview and manual matrix math for the numeric output
• 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 Gradients tool to understand the single-file structure, then inspect tools.json for the registration entry. Add color-picker.html with the listed conversion outputs, palette strip, copy controls, and CSS variable snippet, and run bun run generate:sections, bun run generate:index, and bun run generate:favicon to regenerate project metadata.