Add auto column width mode

[kdkavanagh-agent]

Summary

• Adds a toggleable auto column width mode (press w) that sizes columns based only on the currently visible rows instead of the full dataset
• Widths update dynamically as rows scroll in/out of view or the terminal is resized
• Only recomputes when the visible row range (scroll_y, dt_height) actually changes — cursor movement within the same visible area skips recomputation
• Toggling off restores the original precomputed full-data widths

Test plan

• 7 unit tests covering toggle, width narrowing, scroll updates, restore on toggle-off, min-width from header name, skip recompute when range unchanged, resize handling
• 4 snapshot tests: auto width off (baseline), auto width on at top, auto width on scrolled to bottom, toggled off after on
• All 46 existing tests pass with no regressions
• make check passes (format, lint, typecheck)

🤖 Generated with Claude Code

[kdkavanagh]

This breaks when all the columns currently displayed have null/no values. The error is:

dt_browser/custom_table.py:627 in render_header_and_table                                                                                                                                                                                                                    │
│                                                                                                                                                                                                                                                                                                                                            │
│   624 │   │   │   │   cursor_col_idx = self.cursor_coordinate.column - self._dt.columns.index(                                                                                                                                                                                                                                             │
│   625 │   │   │   │                                                                                                                                                                                                                                                                                                                        │
│   626 │   │   │   │   cols_before_selected: list[str] = visible_cols[0:cursor_col_idx]                                                                                                                                                                                                                                                     │
│ ❱ 627 │   │   │   │   sel_col = visible_cols[cursor_col_idx]                                                                                                                                                                                                                                                                               │
│   628 │   │   │   │   cols_after_selected = visible_cols[cursor_col_idx + 1 :]                                                                                                                                                                                                                                                             │
│   629 │   │   │   │                                                                                                                                                                                                                                                                                                                        │
│   630 │   │   │   │   rend = rend.select(      

[kdkavanagh-agent]

Fixed! The issue was that when auto-width recalculates narrower widths (e.g. for all-null visible columns), the set of visible_cols in render_header_and_table can shift such that cursor_col_idx exceeds the list bounds.

Fix: Added a clamp on cursor_col_idx at line 625 to keep it within [0, len(visible_cols) - 1].

Test: Added test_auto_width_all_null_columns that creates a DataFrame with all-null string columns and enables auto-width mode — confirms no crash and widths are at least header-name length.

All 47 tests pass, make check clean.