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., 500 rows)
• The user-facing message should be clear but not alarming

[IwoA]

I strongly insist on implementing this feature! I have a database with over 8 million records to analyse. Loading all of them takes ages.
I initially thought that simply replacing the standard sql query "SELECT * FROM" would be enough, but apparently, the function doesn't work that way. By the way, this behavior is not very intuitive.
My suggestion is to replace the max_rows parameter with an init_query parameter with a default value "SELECT * FROM table LIMIT 500;". This approach would be much more flexible.