docs: add full user manual and align contributing/changelog

Summary

• Added a full project-specific user manual at docs/USER_MANUAL.md.
• Added a direct manual link in README.md.
• Reworked CONTRIBUTING.md with:
  • cleaner structure
  • updated env/config guidance
  • current type-check command (npx tsc -b)
  • changelog update process
  • updated MR checklist
• Updated CHANGELOG.md ([Unreleased]) with the documentation changes.

Files Changed

• docs/USER_MANUAL.md (new)
• README.md
• CONTRIBUTING.md
• CHANGELOG.md

Validation

• Documentation reviewed for consistency with current app behavior and commands.
• Verified docs/ contains only USER_MANUAL.md.

Closes #12 (closed)