Add Code settings group: custom admin CSS/JS + header/footer scripts

New Settings > Code group (Custom CSS & JS + Header & footer scripts) driven
by admin/src/Code/CodeInjection.php:
- Admin CSS  → <style> in the admin <head> (</style> stripped to prevent breakout)
- Admin JS   → <script> in the admin footer (</script> neutralized)
- Header scripts → printed verbatim in the front-end <head>
- Footer scripts → printed verbatim before the front-end </body>

Header/footer scripts are raw admin-authored markup by design (analytics,
meta, etc.), settable only by manage_options users on the settings page.
SettingsField gains a `mono` flag for monospace textareas (no CodeMirror
dependency). All four injection points verified on PHP 8.5.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

app/src/pages/settings/components/SettingsField.vue

@@ -138,6 +138,7 @@ const installCompanion = async () => {
       <textarea
         v-else-if="field.type === 'textarea'"
         class="field__input field__textarea"
+        :class="{ 'field__textarea--mono': field.mono }"
         :placeholder="field.placeholder || ''"
         :rows="field.rows || 6"
         :value="val || ''"
@@ -346,6 +347,11 @@ const installCompanion = async () => {
 }
 .field__input:focus { outline: none; border-color: var(--aurora-accent); }
 .field__textarea { resize: vertical; font-family: inherit; line-height: 1.5; }
+.field__textarea--mono {
+  font-family: ui-monospace, SFMono-Regular, Menlo, Consolas, 'Liberation Mono', monospace;
+  font-size: 0.8125rem;
+  tab-size: 2;
+}
 
 .field__color { display: flex; align-items: center; gap: 10px; }
 .field__color input[type='color'] {