Never fail silently on PDF export; add EXPORTPDF <path> (#369).
The export flow handed off to an async rfd save dialog and mapped a None
result straight to Task::none(). On Linux None also means the dialog never
opened (broken XDG portal / missing zenity), and rfd reports that only via
log, so PDF export could end in total silence. The dialog-None arms now
print a message pointing at EXPORTPDF <path>, and EXPORT/EXPORTPDF take an
optional path that exports straight to a file with no dialog — the
workaround where the native dialog can't open, and a headless entry point.
Conflict: the fix (display.rs, file.rs, mod.rs, COMMANDS.md) auto-merged;
only the additive test block in automation.rs conflicted. Resolved by
keeping main's tests and adding the two #369 tests.
# Conflicts:
# src/app/automation.rs
5b4a6719aa