feat: refresh MiniMax defaults (M3) and add global/China endpoint selection - #1171

Reason: Refresh the MiniMax provider defaults to MiniMax-M3 and add explicit global/China endpoint selection.

Changes

• pdf2zh/translator.py: bump the default MINIMAX_MODEL to MiniMax-M3 and add a configurable MINIMAX_BASE_URL env (default: global endpoint https://api.minimax.io/v1), following the repository's existing configurable base-URL convention so the China endpoint https://api.minimaxi.com/v1 can be selected without code changes.
• docs/ADVANCED.md, docs/README_ja-JP.md, docs/README_ko-KR.md, docs/README_zh-TW.md: update the provider table to document the new MiniMax-M3 default, the still-selectable MiniMax-M2.7 model, and both the global and China endpoints.
• test/test_translator.py: add TestMiniMaxTranslator covering the default model and global endpoint, explicit MiniMax-M2.7 selection, and China endpoint selection.

Why

The default was still MiniMax-M2.7 and the base URL was hardcoded to the global endpoint only. This refreshes the default to the current flagship MiniMax-M3, exposes both regional endpoints through a single env var, and documents both model routes so users can pick the model and region that fit their account.

Testing

• python -m pytest test/test_translator.py — 12 passed
• python -m flake8 pdf2zh/translator.py test/test_translator.py — clean