fix: preserve runs view zoom

[AssaadHalabi]

Fixes #4800.

Summary

• Preserve the current canvas viewport when entering runs mode so zoom does not reset.
• Keep the runs viewport across mode toggles, and only refit when the selected run's node set changes.
• Add unit coverage for the runs viewport refit decision.

Tests

• git diff --check
• Not run: make format.js, make check.build.ui (Docker engine and local npm/make are unavailable in this environment).

[superplanehq-integration]

👋 Commands for maintainers:

• /sp start - Start an ephemeral machine (takes ~30s)
• /sp stop - Stop a running machine (auto-executed on pr close)

[shiroyasha]

Thank you for opening this PR, and for contributing to the project!
It looks like the issue this PR was addressing has already been fixed.

I’ll close this PR for now. Thanks again for taking the time to contribute.