feat(xray): add loopback outbound protocol

#4185
Surface xray-core's loopback outbound in the Outbounds form so users
can re-route already-processed traffic back into a named inbound for
secondary routing (e.g. splitting TCP/UDP from one ingress). The
inboundTag field is an autocomplete over existing inbound tags, with
free-text fallback for inbounds defined outside the panel. Loopback
outbounds are excluded from the connectivity test since they have no
network endpoint.

frontend/src/pages/xray/OutboundFormModal.vue

@@ -34,6 +34,7 @@ const props = defineProps({
   open: { type: Boolean, default: false },
   outbound: { type: Object, default: null },
   existingTags: { type: Array, default: () => [] },
+  inboundTags: { type: Array, default: () => [] },
 });
 
 const emit = defineEmits(['update:open', 'confirm']);
@@ -199,6 +200,7 @@ const isBlackhole = computed(() => proto.value === Protocols.Blackhole);
 const isDNS = computed(() => proto.value === Protocols.DNS);
 const isWireguard = computed(() => proto.value === Protocols.Wireguard);
 const isHysteria = computed(() => proto.value === Protocols.Hysteria);
+const isLoopback = computed(() => proto.value === Protocols.Loopback);
 
 function regenerateWgKeys() {
   if (!outbound.value?.settings) return;
@@ -311,6 +313,16 @@ function regenerateWgKeys() {
             </a-form-item>
           </template>
 
+          <!-- ============== Loopback ============== -->
+          <template v-if="isLoopback">
+            <a-form-item label="Inbound tag">
+              <a-auto-complete v-model:value="outbound.settings.inboundTag"
+                :options="inboundTags.map((tag) => ({ value: tag }))"
+                :filter-option="(input, option) => option.value.toLowerCase().includes(input.toLowerCase())"
+                placeholder="tag of an existing inbound to re-route into" />
+            </a-form-item>
+          </template>
+
           <!-- ============== DNS ============== -->
           <template v-if="isDNS">
             <a-form-item :label="t('pages.inbounds.network')">