DejaVu Sans subset to 5 glyphs (1.9 KB) ensures toolbar arrows and
sort triangles render correctly on all systems. The font is loaded
via a new setup_efd_fonts() helper that users call during app init.

- Back/Forward buttons use visible text labels ('← Back', '→ Forward')
- Checkboxes carry entry name as accessible label via widget_info
- ArrowUp/ArrowDown moves keyboard focus through entries
- Space toggles selection on focused entry
- Enter navigates into focused directory or toggles file selection
- Focused row gets a distinct background tint (separate from hover)

- Click any column header in Detailed view to sort by that column
- Click again to reverse direction
- Directories always stay on top regardless of sort column
- Raw byte count (size_raw) added to FileEntry for numeric size sort
- Column headers now use Sense::click() and return the clicked column