ci(release): check version metadata consistency

[terminalchai]

Summary

• Add npm run check:versions to compare release versions in package.json, both root version fields in package-lock.json, src-tauri/Cargo.toml, the OffPDF package in src-tauri/Cargo.lock, and src-tauri/tauri.conf.json.
• Run the check in pull-request CI and report exact metadata fields and values on mismatch.
• Add 11 CLI regression tests and document the check in the contributor guide.

Why

Closes #68. The check catches source metadata drift without consulting Git tags or comparing unrelated dependency versions. smol-toml is a development-only dependency for parsing Cargo metadata correctly, including comments and quoted strings.

Validation

• npm run build
• npm test (283 tests across 32 files)
• cargo check --manifest-path src-tauri/Cargo.toml (not run; no Rust code changes)
• npm ci and npm run check:versions passed against the current 0.3.1 metadata.
• Regression tests exercise the CLI exit status and diagnostics for mismatches in every metadata file, the nested npm lockfile version, missing versions, a missing OffPDF Cargo entry, and malformed JSON/TOML.
• Existing dependency audit and build-size warnings remain outside this change.

Privacy Checklist

• This keeps OffPDF usable offline.
• This does not upload, log, or transmit user files.
• New dependencies or bundled binaries have compatible licenses (smol-toml: BSD-3-Clause, development-only; no bundled binaries added).

[McanKul]

Thanks @terminalchai — this is a solid implementation. I verified the version check, all 283 tests, and the production build locally; both CI jobs are green too. The field-level diagnostics and regression coverage are especially useful. We'd be glad to have you take on another issue anytime.