tun: dual MTU, gateway, DNS, auto routing

Change TunSettings to support separate IPv4/IPv6 MTU values and add gateway, DNS, autoSystemRoutingTable and autoOutboundsInterface properties. Introduces _normalizeMtu to accept legacy single-value or array forms and provide sensible defaults. Update fromJson/toJson to handle new fields and preserve backward compatibility. Update tun form UI to expose MTU IPv4/IPv6 inputs, Gateway/DNS tag selects, Auto Routing Table and Auto Outbounds input.

web/html/form/protocol/tun.html

@@ -22,17 +22,43 @@
         <template slot="title">
           <span>{{ i18n "pages.xray.tun.mtuDesc" }}</span>
         </template>
-        MTU
+        MTU IPv4
         <a-icon type="question-circle"></a-icon>
       </a-tooltip>
     </template>
     <a-input-number
-      v-model.number="inbound.settings.mtu"
+      v-model.number="inbound.settings.mtu[0]"
       :min="1"
       :max="9000"
       placeholder="1500"
     ></a-input-number>
   </a-form-item>
+  <a-form-item label="MTU IPv6">
+    <a-input-number
+      v-model.number="inbound.settings.mtu[1]"
+      :min="1"
+      :max="9000"
+      placeholder="1280"
+    ></a-input-number>
+  </a-form-item>
+  <a-form-item label="Gateway">
+    <a-select
+      mode="tags"
+      v-model="inbound.settings.gateway"
+      :style="{ width: '100%' }"
+      :token-separators="[',']"
+      placeholder="IPv4/IPv6 gateway"
+    ></a-select>
+  </a-form-item>
+  <a-form-item label="DNS">
+    <a-select
+      mode="tags"
+      v-model="inbound.settings.dns"
+      :style="{ width: '100%' }"
+      :token-separators="[',']"
+      placeholder="DNS servers"
+    ></a-select>
+  </a-form-item>
   <a-form-item>
     <template slot="label">
       <a-tooltip>
@@ -49,5 +75,20 @@
       placeholder="0"
     ></a-input-number>
   </a-form-item>
+  <a-form-item label="Auto Routing Table">
+    <a-select
+      mode="tags"
+      v-model="inbound.settings.autoSystemRoutingTable"
+      :style="{ width: '100%' }"
+      :token-separators="[',']"
+      placeholder="e.g. vpn, proxy"
+    ></a-select>
+  </a-form-item>
+  <a-form-item label="Auto Outbounds">
+    <a-input
+      v-model.trim="inbound.settings.autoOutboundsInterface"
+      placeholder="auto"
+    ></a-input>
+  </a-form-item>
 </a-form>
 {{end}}