fix: warn when only --delete-excluded is enabled

[KiryuuLight]

--delete-excluded deletes files on its own — it does not need --delete — but both delete gates only ever checked flags.delete. A task with just that flag enabled showed no ⚠ in the run confirmation and got no prompt when the flag was switched on, so it could remove files at the destination with nothing warning you.

Confirmed against rsync 3.4.3: rsync -a --delete-excluded --exclude='*.log' src/ dst/, with no --delete, removed both the file missing from the source and the excluded one.

Task::destructive() now covers both flags and backs the run confirmation as well as the flag toggle. The toggle alert previously hardcoded --delete in its text and set flags.delete directly; it now carries the field index and renders the real flag name, so switching on del-excl prompts for --delete-excluded. The alert fires when a task becomes destructive rather than once per flag, so adding the second delete flag to an already-destructive task does not prompt a second time.

The confirmation list reads better too: flags are spelled out in full rather than abbreviated, aligned in a column sized to the widest task name, and the warning line is centred and capitalised.

fmt, clippy -D warnings, build and 71 tests clean.