Add Free Browser Tools section for Mandarin learners

[daligao]

Summary

Added a new "Free Browser Tools" section to the Mandarin.md page with 8 free, offline browser tools designed specifically for Mandarin learners:

1. Pinyin Annotator — paste Chinese text, get pinyin instantly above each character
2. HSK Text Analyzer — color-code any text by HSK 1–6 level, see comprehension %
3. Chinese Reading Lab — 10 historical decision scenarios with comprehension questions
4. Chengyu Stories — 20 Chinese idioms with origin stories + modern usage quizzes
5. Mandarin Flashcards — HSK 1–3 spaced repetition, 400+ words
6. Chinese Writing Toolkit — 11 types of writing (emails, cover letters, etc.) with templates
7. Chinese Antonyms Game — match 380 opposite-word pairs across 3 levels
8. Chinese Measure Words Game — sort nouns into correct measure word buckets

All tools are:

• Free to use
• No login required
• Work offline in any browser
• Open source with GitHub repos

This fills a gap in the existing Mandarin section, which currently focuses on videos/podcasts/grammar but lacks interactive practice tools.

🤖 Generated with Claude Code