Add Linux Menu MMR OCR support and improve recognition

[iQworte]

Summary

• Add cross-platform screen capture for Menu MMR OCR on Linux (node-screenshots, slop/slurp region picker, grim Wayland fallback, manual coordinates in UI).
• Improve OCR preprocessing with dual-pass image enhancement for bright MMR digits and low-contrast rating label text.
• Require mmr/rating marker in recognized text before updating MMR; parse numbers per line to avoid false positives.
• Document Linux OCR setup in README and add screen-capture unit tests.

Test plan

• npm run test:menu-mmr-ocr
• npm run test:screen-capture
• npm run check
• On Windows: pick OCR region, verify MMR updates in Dota main menu
• On Linux X11/Wayland: verify region picker or manual coords and OCR update flow

[Seno47]

Thanks! I merged the equivalent patch into master and released it as v1.5.8: https://github.com/Seno47/DotaStreamKit/releases/tag/v1.5.8

I also added a few release hardening fixes on top: macOS uses manual coordinates instead of a non-existent native picker, slurp output parsing is fixed, and shell scripts are normalized to LF endings.