feat(json): swap raw textareas for a CodeMirror 6 JsonEditor

A new JsonEditor.vue component wraps CodeMirror 6 + lang-json with
line numbers, JSON syntax highlighting, bracket matching, code
folding, search (Ctrl+F), undo/redo, lint (red squiggle and gutter
icon on invalid JSON), tab indent, and line wrapping. It is wired
into the four raw-JSON spots that previously used <a-textarea
class="json-editor">: the Xray Advanced Template tab, the Outbound
JSON tab, the Balancer Observatory pane, and the Inbound Advanced
tab (settings / streamSettings / sniffing).

Chrome colors are driven by EditorView.theme so they win the
specificity fight cleanly against CodeMirror's own injected styles.
A single buildDarkTheme() factory yields a Dark+ palette (#1e1e1e
background, #252526 active line, #2d2d30 panels) for the regular
dark mode and a near-black variant (#0a0a0a / #141414 / #1f1f1f
border) for ultra-dark — both pair with oneDarkHighlightStyle for
the syntax colors. Light mode stays on basicSetup's default.

CodeMirror lazy-loads as a ~17 kB gzipped chunk that only appears
on the Xray/Inbounds bundles.

frontend/src/pages/xray/BalancersTab.vue

@@ -10,6 +10,7 @@ import {
 import { Modal } from 'ant-design-vue';
 
 import BalancerFormModal from './BalancerFormModal.vue';
+import JsonEditor from '@/components/JsonEditor.vue';
 
 const { t } = useI18n();
 
@@ -305,8 +306,7 @@ const obsText = computed({
           <a-radio-button v-if="hasObservatory" value="observatory">Observatory</a-radio-button>
           <a-radio-button v-if="hasBurstObservatory" value="burstObservatory">Burst Observatory</a-radio-button>
         </a-radio-group>
-        <a-textarea v-model:value="obsText" :auto-size="{ minRows: 8, maxRows: 24 }" spellcheck="false"
-          class="json-editor" />
+        <JsonEditor v-model:value="obsText" min-height="220px" max-height="480px" />
       </template>
     </template>
 
@@ -330,9 +330,4 @@ const obsText = computed({
   color: #ff4d4f;
 }
 
-.json-editor {
-  font-family: ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, monospace;
-  font-size: 12px;
-  margin-top: 8px;
-}
 </style>