Add Simplified Chinese to the browser extension

[myml]

What

Follow-up to the review of #71. The admin app gained Simplified Chinese there, and this extension's README promised the admin app's language set, so a browser reporting zh-CN met a Chinese dashboard next to an English popup.

• src/i18n/zh.ts carries the 69 keys the popup, the form, the error messages and the options page read.
• zh joins SUPPORTED_LANGUAGES. The table map beside it is typed over that list, so a language registered without a table fails the typecheck rather than resolving to undefined at the first t().
• The language is the browser's, read by its primary subtag, so zh-CN, zh-SG and zh-Hans all resolve to zh. A script variant the set does not carry, zh-TW or zh-HK, also resolves to zh and reads Simplified: the extension carries one Chinese table and has no picker to choose another. detectLanguage() documents that, and a test pins it as a decision rather than an accident. A Traditional table would have to be matched before the primary subtag is taken, which this does not do.
• The README no longer enumerates the languages. It points at SUPPORTED_LANGUAGES, which is also where a new locale is registered, so the claim cannot drift from the code.

Tests

• The parity suites iterate SUPPORTED_LANGUAGES instead of naming id and sv, so this locale and any later one are covered the day they register. The accepts en, id, sv case became every-language acceptance plus an assertion on the exact array, and a new case checks that each table declares its own code in _lang.
• A new popup case renders with navigator.language set to zh-CN and asserts the Chinese labels, which is the whole path from the browser to the strings on screen. Every other case in that file runs against the en-US pin in tests/setup.ts.
• Mutating the array to drop zh fails four cases; dropping a placeholder from the zh table fails the placeholder parity case.
• yarn test: 9 files, 165 tests pass. yarn typecheck: clean. yarn build, scripts/verify-build.mjs: both targets build, 7 referenced files present, 45.1 KB zips.

The Chinese labels are the widest strings the popup and options page carry, so RELEASE.md gains a step that checks them for clipping on those two surfaces before a tag.