fix(outbounds): support proxyProtocol on freedom outbound

Xray's freedom outbound accepts a numeric proxyProtocol (0 disabled,
1 or 2 for the PROXY protocol version), but the panel had no field for
it and the typed form adapter dropped the key on save — so a value set
via the JSON editor disappeared the moment the outbound was saved.

Model proxyProtocol through the freedom wire schema, the form schema,
and both adapter directions (clamped to 0/1/2, omitted from the wire
when 0), and add a Select (none / v1 / v2) to the freedom section of
the outbound form. Add round-trip test coverage and the proxyProtocol
label across all locales.

Closes #4486

frontend/src/pages/xray/OutboundFormModal.tsx

@@ -664,6 +664,15 @@ export default function OutboundFormModal({
                         <Form.Item label={t('pages.xray.outboundForm.redirect')} name={['settings', 'redirect']}>
                           <Input />
                         </Form.Item>
+                        <Form.Item label={t('pages.xray.outboundForm.proxyProtocol')} name={['settings', 'proxyProtocol']}>
+                          <Select
+                            options={[
+                              { value: 0, label: `(${t('none')})` },
+                              { value: 1, label: 'v1' },
+                              { value: 2, label: 'v2' },
+                            ]}
+                          />
+                        </Form.Item>
 
                         <Form.Item label={t('pages.xray.outboundForm.fragment')} shouldUpdate noStyle>
                           {() => {