Release v1.3.1 - FEN Validation Fix & Install Guide Cleanup

🧠 Highlights

• Restores the front-end validation logic so the Analyze Position button re-enables as soon as you type or paste a valid FEN.
• Reset now works again and console logging around validation/reset flows helps future debugging.

🛠️ Fixes & improvements

• Repaired the inline JavaScript so Jinja2 no longer injects syntax errors.
• Cleaned up the helper automation scripts (step1.py–step3.py) so they target app.py.
• Rewrote INSTALL.md with proper Markdown code blocks, clarified the Python 3.11 requirement, and refreshed the Windows shortcut instructions.
• Bumped all project metadata (version.py, setup.py, pyproject.toml, README) to 1.3.1 for consistency.

🧪 Verification

• Manual browser test of the updated UI
• python -m compileall app.py