Turn the outbound sockopt dialerProxy free-text input into a searchable Select populated with the other outbound tags, so users can build a proxy chain (route one outbound through another) without typing tags by hand. The list excludes the current outbound, so self-reference cycles cannot be selected. A tooltip and placeholder explain the chaining concept. Adds dialerProxyPlaceholder and dialerProxyHint to all 13 locales.
Closes#4446