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/OutboundFormModal.vue

@@ -21,6 +21,7 @@ import {
   DNSRuleActions,
 } from '@/models/outbound.js';
 import FinalMaskForm from '@/components/FinalMaskForm.vue';
+import JsonEditor from '@/components/JsonEditor.vue';
 
 const { t } = useI18n();
 
@@ -988,8 +989,7 @@ function regenerateWgKeys() {
               <a-button>Convert</a-button>
             </template>
           </a-input-search>
-          <a-textarea v-model:value="advancedJson" :auto-size="{ minRows: 14, maxRows: 30 }" spellcheck="false"
-            class="json-editor" />
+          <JsonEditor v-model:value="advancedJson" min-height="360px" max-height="600px" />
         </a-space>
       </a-tab-pane>
     </a-tabs>
@@ -1032,11 +1032,6 @@ function regenerateWgKeys() {
   opacity: 0.85;
 }
 
-.json-editor {
-  font-family: ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, monospace;
-  font-size: 12px;
-}
-
 /* AD-Vue 4 renders a-checkbox children inside a-checkbox-group as
  * inline-block, but inside a narrow form wrapper they can wrap
  * inconsistently. Force a clean horizontal row with even gaps. */