Add Gracestack Dev Tools — 14 free browser-based developer tools

[cedendahlkim]

Summary

Adds Gracestack Dev Tools to the Productivity & Misc section.

14 free browser-based tools, all client-side (no upload to server):

• Image Optimizer (compress PNG/JPEG/WebP, quality slider, before/after comparison)
• CSS Gradient Generator (linear/radial, 10 presets, CSS export)
• JSON Formatter (prettify, minify, validate)
• Regex Tester (real-time matching, capture groups, flags)
• JWT Decoder (decode and inspect JWT tokens locally)
• Markdown Previewer (live preview with marked.js)
• Text Diff Checker (LCS-based diff with color coding)
• Color Converter (HEX↔RGB↔HSL, live swatch)
• Code Formatter (JS/JSON/CSS beautify + minify)
• JSON Schema Validator (draft-07, sample schemas)
• Color Contrast Checker (WCAG AA/AAA compliance)
• Lorem Ipsum Generator (paragraphs/sentences/words, HTML export)
• Base64 Encoder/Decoder + UUID Generator + URL Encoder + Timestamp Converter

All tools: no ads, no signup, no data collection, fully client-side.