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/test/outbound-form-adapter.test.ts

@@ -235,16 +235,26 @@ describe('outbound-form-adapter: round-trip', () => {
       settings: {
         domainStrategy: 'UseIPv4',
         redirect: '1.1.1.1',
+        proxyProtocol: 2,
         fragment: { packets: 'tlshello', length: '100-200' },
       },
     }));
     expect(filled.settings).toMatchObject({
       domainStrategy: 'UseIPv4',
       redirect: '1.1.1.1',
+      proxyProtocol: 2,
       fragment: { packets: 'tlshello', length: '100-200' },
     });
   });
 
+  it('freedom omits proxyProtocol when disabled (0)', () => {
+    const round = formValuesToWirePayload(rawOutboundToFormValues({
+      protocol: 'freedom',
+      settings: { proxyProtocol: 0 },
+    }));
+    expect((round.settings as { proxyProtocol?: number }).proxyProtocol).toBeUndefined();
+  });
+
   it('mux is only emitted when enabled AND protocol/network/flow allow it', () => {
     // Disabled mux: omitted
     const disabled = formValuesToWirePayload(rawOutboundToFormValues({