Fix MLWM password, visibility, and UI i18n

[CEQ151]

Summary

• Restore neural image-watermark password protection with keyed-v2 payload masking.
• Add three neural visual-strength profiles: invisible, balanced, robust.
• Polish image watermark/readability UI and add lightweight system/zh-CN/en i18n for main desktop flows.

Validation

• python -m unittest discover -s blind_watermark\tests
• npm run typecheck
• npm run build
• Rebuilt resources/bin/bwm_helper.exe locally with PyInstaller.
• EXE smoke: neural balanced embed/extract succeeds with password 2468; extraction fails with wrong password 2469.
• ONNX smoke: invisible/balanced/robust clean extraction succeeds; balanced JPEG75/JPEG50/resize50 succeeds on local natural sample.

Notes

• Bundled helper output is ignored by git, so the PR keeps source changes traceable while the local desktop build already has the refreshed exe.
• Existing untracked local files under docs/mlwm_dataset_sources.md, monitor_samples/, and tools/ were left untouched.