Add automated version update script and workflow

[jgeudens]

Summary

• Adds scripts/update_version.sh — a standalone script that updates all hardcoded version references in index.html (2× download buttons, 1× footer copyright) and all fields in updater/version.json. Takes TAG_NAME HTML_URL PUBLISHED_AT as positional args; can be run locally too.
• Adds .github/workflows/update-version.yml — runs daily at 06:00 UTC (and on manual dispatch). Fetches the latest non-prerelease release from ModbusScope/ModbusScope, compares tag_name against updater/version.json, and if a new version is found: calls the script, creates a update-version-X.Y.Z branch, commits the changes, and opens a pull request to main.

Test plan

• Merge this PR
• Manually trigger Actions → Update version → Run workflow — with the current version (4.2.1) already in version.json, no PR should be created
• To test the update path: temporarily change tag_name in updater/version.json to an older value (e.g. 4.2.0), trigger again, and verify a update-version-4.2.1 PR is opened with all 3 index.html occurrences and version.json updated
• Verify the script works locally: ./scripts/update_version.sh 4.3.0 https://github.com/ModbusScope/ModbusScope/releases/tag/4.3.0 2026-06-01T00:00:00Z

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