Vite migration (3/3): check-ui guards and small follow-ups #1581

Part of #1578. Depends on #1580. Not required to boot.

Change. Three guard checks for behaviour a successful build does not demonstrate, wired as make check-ui: script/check-built-assets.sh builds the frontend, runs TestGetStyleResolvesBuiltAssets, and under --self-check rewrites the built index.html into shapes the server cannot use and asserts the check fails on each; ui/scripts/check-locale-resolution.js starts the real dev server and requires a non-default language to resolve through the module graph and to be fetchable over HTTP; ui/scripts/check-plugin-i18n-order.js registers a plugin translation bundle before and after i18next initialises and asserts the translations land. All three fail closed: preset failure, bounded server create, listen and close, a whole-run watchdog. Plus ui/scripts/locale-probe.js, the small CSS fix (Comment/index.scss declaration order) and the pages/403 route repoint. The one-line Editor/index.scss custom-property fix that #1567 also carried is not here: the new CSS minifier rejects the invalid declaration outright and fails the build, so it had to ride in step 2.

Done when. make check-ui is green on all three targets on dev plus steps 1 and 2 plus this change, and ./script/check-built-assets.sh --self-check reports every fixture failing as intended.