docs(auth): say no datasets are shared when accepting the license (ENG-1020)

[eddiebergman-priorlabs]

Issue

Internal ticket ENG-1020.

Motivation and Context

Some users hold back from accepting the model license because they think it shares their datasets with Prior Labs. It doesn't: the login sends only the API key and license version, and once the weights are downloaded TabPFN runs offline.

This puts the note in one shared header, LICENSE_ACCEPTANCE_HEADER in errors.py. The browser and headless login prompts and the license error messages (TABPFN_NO_BROWSER, no terminal, the default TabPFNLicenseError) all use it, so CI users see it too. The README FAQ on getting access gets the same sentence.

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Public API Changes

• No Public API changes
• Yes, Public API changes (Details below)

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How Has This Been Tested?

tests/test_browser_auth.py passes (48 tests), with assertions that both the browser and headless prompts include the header. ruff check and format are clean.

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Checklist

• The changes have been tested locally.
• Documentation has been updated (if the public API or usage changes).
• A changelog entry has been added (see changelog/README.md), or "no changelog needed" label requested.
• The code follows the project's style guidelines.
• I have considered the impact of these changes on the public API.

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🤖 Generated with Claude Code

Part of ENG-1020