fix: disable balancer fallbackTag for random / roundRobin strategies

Xray-core's RandomStrategy and RoundRobinStrategy register a pending
dependency on the Observatory feature whenever fallbackTag is non-empty.
Since the panel only provisions observatory for leastPing / leastLoad
balancers, picking roundRobin with a fallbackTag caused xray to fail
boot with "not all dependencies are resolved". Disable the fallback
field for the two strategies that cannot resolve it, and strip
fallbackTag from the wire balancer as a defensive backstop for users
who edit the JSON template directly.

frontend/src/pages/xray/BalancerFormModal.vue

@@ -61,6 +61,16 @@ const isValid = computed(
   () => !tagEmpty.value && !duplicateTag.value && !emptySelector.value,
 );
 
+const fallbackSupported = computed(
+  () => form.strategy === 'leastPing' || form.strategy === 'leastLoad',
+);
+
+watch(() => form.strategy, (next) => {
+  if (next !== 'leastPing' && next !== 'leastLoad') {
+    form.fallbackTag = '';
+  }
+});
+
 const tagValidateStatus = computed(() => {
   if (tagEmpty.value) return 'error';
   if (duplicateTag.value) return 'warning';
@@ -111,8 +121,9 @@ const okText = computed(() =>
         </a-select>
       </a-form-item>
 
-      <a-form-item label="Fallback">
-        <a-select v-model:value="form.fallbackTag" allow-clear>
+      <a-form-item label="Fallback"
+        :help="fallbackSupported ? '' : 'Available only with Least ping / Least load'">
+        <a-select v-model:value="form.fallbackTag" allow-clear :disabled="!fallbackSupported">
           <a-select-option v-for="tag in ['', ...outboundTags]" :key="tag || '__empty'" :value="tag">
             {{ tag || `(${t('none')})` }}
           </a-select-option>

frontend/src/pages/xray/BalancersTab.vue

@@ -145,10 +145,11 @@ function syncObservatories() {
 }
 
 function buildWireBalancer(form) {
+  const supportsFallback = form.strategy === 'leastPing' || form.strategy === 'leastLoad';
   const out = {
     tag: form.tag,
     selector: [...form.selector],
-    fallbackTag: form.fallbackTag,
+    fallbackTag: supportsFallback ? form.fallbackTag : '',
   };
   if (form.strategy && form.strategy !== 'random') {
     out.strategy = { type: form.strategy };