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

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.

Start with make check-ui and inspect script/check-built-assets.sh, ui/scripts/check-locale-resolution.js, ui/scripts/check-plugin-i18n-order.js, and ui/scripts/locale-probe.js. Review the Comment/index.scss declaration order and pages/403 route, then run the three checks on dev plus steps 1 and 2. Done means make check-ui is green and ./script/check-built-assets.sh --self-check reports every fixture failing as intended.

Written by the indexing model from the issue text.