Add a Simplified Chinese interface catalogue

781 strings, machine-made and marked Beta, on the same terms as the five
Phase 1 languages: the report link stands in for the native speaker we
do not have, and a missing entry falls back to English, so deleting a bad
line is a valid fix.

Two things differ in kind from the European catalogues, and the file
header records both so a later editor does not undo them.

Plurals: there are none. Intl.PluralRules("zh-Hans") returns `other` for
every number, so each counted string carries one form. Supplying `one`,
`few` or `many` would be filling in a distinction the language does not
draw, and none of them would ever be selected.

Script: this is Simplified, and the tag says so. A Traditional catalogue
would be a separate file rather than a character conversion of this one —
the vocabulary differs as much as the script does (软件/軟體, 文件/檔案),
and converting characters alone produces text that is readable and
obviously foreign.

Verified against the mock server: role folders localise and custom ones
are left alone, dates and the calendar follow the language, and the
catalogue code-splits into its own 41 kB chunk.

web/src/lib/languages.ts

@@ -42,6 +42,7 @@ export const UI_LANGUAGES: readonly UiLanguage[] = [
   { tag: "fr", name: "Français", beta: true },
   { tag: "nl", name: "Nederlands", beta: true },
   { tag: "pt-BR", name: "Português (Brasil)", beta: true },
+  { tag: "zh-Hans", name: "简体中文", beta: true },
 ];
 
 /** Where to report a bad translation. Beta languages depend on it. */