fix(list): autofit columns on resize-handle double-click

[JoeJoeflyn]

Description

Double-clicking a List-view column resize handle was a no-op: register_list_column_cell applies the column's fixed width_request to every registered cell, so measure() inside max_child_natural_width echoed the current width back, and all row/heading labels cap natural width at one character via max_width_chars(1). The autofit could therefore never change the width.

This measures each registered cell with both constraints lifted (list_cell_content_width clears the cell's width_request and descendant labels' max_width_chars, measures, then restores them), so double-click now expands the column to fit its widest content — matching the Miller-columns behavior added in #114. The same code path serves the FileChooser dialog, which is fixed identically.

Also includes a separate style: commit applying rustfmt to files that drifted on main.

Visual evidence

screenrecording-2026-09-18_18-14-31.mp4

Pending — needs owner upload. gh cannot attach images; please drag these into the editor:

• target/pr-captures/autofit-before.png — Name column manually narrowed, long filename ellipsized
• target/pr-captures/autofit-after.png — after double-click on the resize handle: column expanded, full filename visible

How to test

1. Switch to List view (Ctrl+3) in a folder containing a file with a long name
2. Drag the Name column's resize handle left to narrow it (or leave at default with a long name)
3. Double-click the resize strip at the right edge of the column header

Expected result: the column expands to fit its widest entry. Works the same in the FileChooser dialog's list view.

Related issue

Closes #1115