Register Dutch, and catch a catalogue nobody can select

nl.ts shipped without an entry in UI_LANGUAGES, so the language was never
offered: the catalogue built, every test passed, the coverage check reported
98%, and the picker did not list it. The entry was added by a text replacement
anchored on the French line, which does not exist on a branch cut from main, so
the replacement was a silent no-op.

A catalogue and a picker entry are two halves of one thing and either half
alone is dead weight, so the checker now verifies both directions -- a
catalogue with no entry, and an entry with no catalogue. Reverting the
one-line fix makes it fail, which is the only way to know a check works.

web/src/lib/languages.ts

@@ -38,6 +38,7 @@ export interface UiLanguage {
 export const UI_LANGUAGES: readonly UiLanguage[] = [
   { tag: "en", name: "English" },
   { tag: "de", name: "Deutsch", beta: true },
+  { tag: "nl", name: "Nederlands", beta: true },
 ];
 
 /** Where to report a bad translation. Beta languages depend on it. */