docs: rewrite project documents in ASD-STE100 English, add screenshots

[Slacker-LLC]

Summary

• README.md is now English, with a screenshot table. The Chinese README moves to README.zh-CN.md.
• Rewrite CONTRIBUTING, SECURITY, THIRD_PARTY_NOTICES, docs/ and scripts/README.md in short, plain English (ASD-STE100 style).
• Translate the issue templates and the PR template.
• Add 8 screenshots in docs/images/ (keyboards and app screens, light and dark), taken from the current debug build.

Stale facts fixed

• SECURITY.md named "the latest 1.x release"; it now names the latest release.
• The README told users to download SHA256SUMS.txt; releases attach only the APK, and the checksum is in the notes.
• Compatibility tests run on API 26, 29, 31 and 34, not only 29 and 31.
• The app UI spec used the old switch size; it now uses 51 × 31 dp.
• The voice document no longer says there is no punctuation model, and no longer describes removed hotwords.

Removed

docs/REPAIR_PLAN.md, docs/BETA4_VOICE.md, docs/REFERENCE_IME_GUIDE.md, docs/CHANGELOG_PRE_1.0.md, docs/MAPPING.md.
They are finished internal plans or release-specific notes. The capability table from MAPPING.md moves to docs/ARCHITECTURE.md. Git history keeps all of them.

Not changed

• CHANGELOG.md entries stay Chinese (release notes, parsed by release_check.py). Only the link to the removed pre-1.0 file is dropped.
• scripts/release_build.sh still generates Chinese release notes. That is code, not documentation.
• App UI strings are Chinese only, so the documents quote the Chinese label with an English note.

Verification

• No broken relative links in the changed Markdown files (checked with a script)
• python3 scripts/release_check.py check and python3 -m unittest discover -s scripts -p 'test_*.py'
• A sentence-length and abbreviation check on the English documents: no sentence over 25 words, no contractions
• No official STE dictionary check was run

No code changes, so the unit tests and lint are not affected.

🤖 Generated with Claude Code