feat: add CI check to require version bump when package files change

Summary

• Adds a check-version-bump CI job that runs on MRs and branch pushes
• Compares changed files against the target branch (main)
• Fails if package files changed but version wasn't bumped

Files monitored

• src/ - source code
• package.json - dependencies and metadata
• webpack.config.js - build configuration
• tsconfig.json - TypeScript configuration

How it works

1. Fetches the target branch (main) for comparison
2. Gets current and target branch versions from package.json
3. Checks if any monitored files have changed
4. If files changed but version is the same, fails with clear error message

Closes #5 (closed)

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