Add interactive CLI and CAdd interactive CLI mode and Chinese READMEhinese README

[666HellUnicorn]

Summary

This PR adds a guided interactive CLI experience for pdfmd and introduces a Simplified Chinese README.

Changes

• add python -m pdfmd entrypoint support via pdfmd/__main__.py
• add guided interactive CLI flow in pdfmd/interactive_cli.py
• add --interactive flag for launching the guided CLI
• add --ui-lang option for interactive CLI language selection
• add OCR language normalization helper in pdfmd/ocr_lang.py
• update CLI entry handling in pdfmd/cli.py
• add README.zh-CN.md
• add language switch links between English and Chinese README files

Why

The current CLI works well for users already familiar with command-line flags, but it is less friendly for users who prefer a guided workflow.

This change aims to:

• lower the barrier to entry for new CLI users
• make OCR-related choices easier to understand during interactive use
• improve accessibility for Chinese-speaking users through localized documentation

Notes

• this PR focuses on CLI usability and documentation only
• OCR engine behavior and rendering heuristics are intentionally not included in this PR
• the Chinese README is added as a separate document rather than replacing the existing English README

Manual testing

• verified python -m pdfmd --version
• verified python -m pdfmd --interactive
• verified interactive CLI flow in Chinese mode with --ui-lang zh
• verified OCR language menu selection in interactive mode
• verified README language switch links