fix(xhttp): stop XMUX maxConcurrency from reverting on save

XHttpXmuxSchema defaulted maxConnections to 6 (added to mirror xray-core
v26.6.27's anti-RKN client default), so load-time hydration backfilled a
non-zero maxConnections onto every config whose saved xmux lacked the
key. Since maxConnections and maxConcurrency are mutually exclusive on
the wire, the save-time exclusivity rule then saw both fields set and
silently deleted the user's maxConcurrency; the missing key came back as
the '16-32' schema default on the next load, so edits appeared to never
save.

Revert the bare schema default to 0 and seed the anti-RKN
maxConnections=6 only when XMUX is freshly toggled on
(XMUX_FRESH_DEFAULTS, with maxConcurrency left blank — xray-core parses
an empty range string as 0), so the two strategies never start out
conflicting. The inbound and outbound XMUX forms now also clear the
opposing field live as soon as the user sets one, so whichever strategy
was actually typed is the one persisted.

Closes #5864

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

@@ -521,6 +521,29 @@ describe('outbound-form-adapter: xhttp xmux toggle', () => {
     const wireXhttp = (back.streamSettings as Record<string, unknown>).xhttpSettings as Record<string, unknown>;
     expect(wireXhttp).not.toHaveProperty('xmux');
   });
+
+  it('keeps a saved maxConcurrency through a load/re-save round-trip', () => {
+    const wire = {
+      ...xmuxWire,
+      streamSettings: {
+        ...xmuxWire.streamSettings,
+        xhttpSettings: {
+          ...xmuxWire.streamSettings.xhttpSettings,
+          xmux: { maxConcurrency: '1-2' },
+        },
+      },
+    };
+    const form = rawOutboundToFormValues(wire);
+    const xhttp = (form.streamSettings as Record<string, unknown>).xhttpSettings as Record<string, unknown>;
+    const xmux = xhttp.xmux as Record<string, unknown>;
+    expect(xmux.maxConcurrency).toBe('1-2');
+    expect(xmux.maxConnections).toBe(0);
+
+    const back = formValuesToWirePayload(form);
+    const wireXhttp = (back.streamSettings as Record<string, unknown>).xhttpSettings as Record<string, unknown>;
+    const wireXmux = wireXhttp.xmux as Record<string, unknown>;
+    expect(wireXmux.maxConcurrency).toBe('1-2');
+  });
 });
 
 describe('outbound-form-adapter: full optional-block round-trip', () => {