feat(outbounds): pick dialerProxy from other outbound tags for proxy chaining

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

frontend/src/pages/xray/outbounds/OutboundFormModal.tsx

@@ -575,7 +575,9 @@ export default function OutboundFormModal({
 
                     {security === 'reality' && realityAllowed && <RealityForm />}
 
-                    {((streamAllowed && network) || !streamAllowed) && <SockoptForm form={form} />}
+                    {((streamAllowed && network) || !streamAllowed) && (
+                      <SockoptForm form={form} outboundTags={existingTags} />
+                    )}
 
                     <FinalMaskForm
                       name={['streamSettings', 'finalmask']}