[nishantmonu51]

Public URLs (/-/share/rill_mgc_...) only grant viewing and exporting, but the Export menu in the dimension table, time dimension detail and pivot views still offered Create scheduled report..., which failed with a 403 when used.

• Expose authContext on RuntimeClient (it was already tracked privately and is "magic" on public URL pages).
• ExportMenu hides the scheduled report option, and skips loading the dialog, when the auth context is "magic".
• Extend the public URL e2e test to assert that the Export menu offers CSV export but not scheduled reports for anonymous viewers.

Steps to reproduce (before this change):

1. Open an explore dashboard in Rill Cloud, click Share → Create public URL, and open the generated link in a private window.
2. Click a dimension to open its dimension table, then open Export.
3. Create scheduled report... is listed; submitting the dialog returns a 403.

Closes APP-955

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!