fix: stop printing from fit/predict and demote routine log records

[ggprior]

fit() and predict() wrote a \r spinner to stdout on every call, and routine events were logged at WARNING, which Python prints to stderr even when the application has configured no logging. In scripts and CI logs the spinner frames end up in captured output; in notebooks the spinner and log lines share one output line. A library should leave stdout and logging configuration to the application.

• Spinner removed. fit() and predict() call the server directly, without the worker thread the spinner needed. TABPFN_CLIENT_CI_MODE, which only disabled the spinner, is removed; setting it is now a no-op.
• "Train/Test set already exists, skipping upload." is logged at DEBUG. It is the normal dedup path, e.g. predicting on the same X twice.
• init() no longer prints "Found existing access token, reusing it for authentication." on every run in a terminal. try_reuse_existing_token() already logs the same event at DEBUG.
• try_connection() no longer logs an ERROR and calls traceback.print_exc() before init() raises its own connection error. The traceback is logged at DEBUG.
• UserDataClient logs through its module logger. It called logging.info() / logging.error(), which run basicConfig() when the root logger has no handlers, so a later basicConfig() in the application did nothing. The error logs that preceded a re-raise are dropped.
• Missing tabpfn / torch for output_type="full" is now a UserWarning via warnings.warn: the caller can act on it and filter it.
• Unused ui helpers are removed: setup_logging() (a basicConfig() helper), header(), info(), progress_bar() and print_logo_small(). None had callers.

Unchanged: the interactive login and signup flows still print, since they are opt-in and need a terminal, and server greeting messages still go through notify().

Not in this PR: the import-time setLevel(WARNING) on the httpx / httpcore loggers, and the server deprecation notice being a DeprecationWarning, which is hidden by default outside __main__.

Generated with Claude Code

https://claude.ai/code/session_018bhEkFCJWSiwyStDihRFMa

[ggprior]

This change is part of the following stack:

• fix: stop printing from fit/predict and demote routine log records #385 ◀

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