v0.6.16

feat: remember the last column layout across runs

The signal columns ([1]/[2]/[3]), box columns (Shift+1/2/3) and section
columns (F1/F2/F3) are now persisted to config.toml ([app] layout_columns /
layout_groups / layout_pages) on every change and restored on the next launch.
A line-based writer (write_layout) merges the keys without disturbing the
theme key, comments or other sections; load_config reads them back (clamped to
1..3) and the app applies the saved layout to every view on mount.

Bumps to 0.6.16.