Add max_rows parameter to .app() for large data handling

[cpsievert]

Summary

Implement a max_rows parameter on the .app() method to handle large datasets gracefully.

Background

The warn_if_large_dataframe() logic was removed in preparation for a better approach to handling large datasets. Instead of just warning developers, we should provide a configurable way to limit displayed data.

Proposed Solution

1. Add a max_rows parameter to the .app() method
2. Use .head(max_rows) on filtered data before displaying
3. Warn the developer (via console/logs) that not all data is shown in the app
4. Provide a sensible message to the app user indicating that not all data is available (e.g., "Showing first N of M rows")

Implementation Notes

• This should be implemented after Polars and Ibis support has landed
• The max_rows parameter should have a sensible default (e.g., 10,000 rows)
• The user-facing message should be clear but not alarming