[DOCS] Restore dev-version banner gate after VitePress migration (#2305)

The VuePress->VitePress migration replaced the `themeConfig.mode === 'dev'`
gate with a heuristic on `site.title`. CI overrides version with a short
SHA (dev) or git tag (prod), so the title never contains "edge"/"dev" and
the banner never renders on live deploys. Surface `meta.mode` in
`themeConfig` and check it directly, mirroring the original behaviour.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/.vitepress/config.mts

@@ -32,6 +32,12 @@ export default defineConfig({
   },
   themeConfig: {
     siteTitle: 'OpenMQTTGateway',
+    // @ts-expect-error custom fields consumed by index.md to render the dev-version banner
+    mode: meta.mode,
+    // @ts-expect-error
+    version: meta.version,
+    // @ts-expect-error
+    repo: meta.theme_config_repo,
     nav: commonNav,
     socialLinks: [
       { icon: 'github', link: `https://github.com/${meta.theme_config_repo}` }