Issue #3593113 by mably: Add per-config checkboxes to the override delete confirmation form

The delete confirmation now lists one checkbox per configuration name, all checked by default. Unchecking a configuration keeps its override, so a single remove trigger can no longer wipe an unrelated override (for example losing Site Information values when clearing account settings that share system.site). Submitting with nothing selected is rejected. Covered by a new functional test.

Closes #3593113