[lint] Enforce custom dark-theme: prefix instead of standard dark: prefix (#5382)

- Add vue/no-restricted-class ESLint rule to catch dark: prefix usage
- Fix existing violation in FormImageUpload.vue (dark: -> dark-theme:)
- Ensures consistent usage of project's custom Tailwind variant
- Violations will fail builds and be caught during development

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <noreply@anthropic.com>

src/components/common/FormImageUpload.vue

@@ -3,7 +3,7 @@
     <div class="flex gap-2 items-center">
       <div
         class="preview-box border rounded p-2 w-16 h-16 flex items-center justify-center"
-        :class="{ 'bg-gray-100 dark:bg-gray-800': !modelValue }"
+        :class="{ 'bg-gray-100 dark-theme:bg-gray-800': !modelValue }"
       >
         <img
           v-if="modelValue"