Add 9 free in-browser tool sites (MathLab, ColorKit, ConverterKit, DateTime, HealthFit, ImgLab, NetChecks, Picker, WordCraft)

[hhammoud]

Adding a family of free, no-signup, in-browser tool sites. Each runs client-side with no account required:

• MathLab (https://mathlab.tools/) — scientific calculator, equation/matrix/fraction solvers, statistics, graphing
• ColorKit Tools (https://colorkit.tools/) — palette generator, picker, gradient maker, hex/RGB converter, WCAG contrast (filename colorkit-tools.md since colorkit.md is taken)
• ConverterKit Tools (https://converterkit.tools/) — unit converters (currency, length, weight, temperature, cooking, area, volume, speed)
• DateTime Tools (https://datetime.tools/) — timezone converter, stopwatch, age calculator, countdown, Unix/Discord timestamps
• HealthFit (https://healthfit.tools/) — BMI/BMR/TDEE/body-fat and pregnancy calculators (educational)
• ImgLab Tools (https://imglab.tools/) — local image converters (HEIC/PNG/WebP), compressor, resizer, favicon generator
• NetChecks (https://netchecks.tools/) — IP/DNS/WHOIS/ping/port/SSL/MAC-vendor lookups
• Picker Tools (https://picker.tools/) — wheel of names, dice, coin flip, random number/word/letter
• WordCraft Text Tools (https://wordcraft.tools/) — fancy text generators, word counter, case converter

Grouped into one PR to keep review simple; happy to split if preferred.