feat: Select multiple subnets for probe and export #12

Problem

I often want to probe or export only part of the subnet inventory. Currently, actions apply to one subnet or the whole inventory.

Proposed change

Allow selecting multiple subnet rows using standard desktop controls: Cmd/Ctrl-click for individual rows and Shift-click for a range. Probe and export actions should then apply to the selected subnets.

Alternatives considered

Probing or exporting each subnet individually, which is repetitive for larger selections.

Scope

• This request fits proxybench's single-window proxy subnet workflow.