Replaces the previously hard‑wired bottom panel model statistics logic in etable with more flexible system.

Key changes:

• New arguments: model_stats (list of attribute names without leading _) and model_stats_labels (optional label overrides).
• Any model attribute can be displayed by listing its name without "_" (e.g. _r2_within -> "r2_within", _N -> "N"). Should therefore also work with future estimation methods and attributes.
• Default (when model_stats=None) reproduces legacy behavior: ["N", "se_type?" if show_se_type, "r2", ("r2_within" if available else "adj_r2")].
• Passing an empty list model_stats=[] suppresses all built‑in stats.

Addresses #983.