i18n: add Traditional Chinese (zh-TW)

Add a complete Taiwan Traditional Chinese catalog, locale selection,
Chinese readout formatting, translation notes and regression coverage.
Seed translations from #724 and #652 and adapt the zh-CN catalog.

Co-authored-by: caxer <[email protected]>
Co-authored-by: tbdavid2019 <[email protected]>
Co-authored-by: Enzo Abyie <[email protected]>
Co-authored-by: EDIT-Aeterna <[email protected]>

docs/CURRENT-STATE.md

@@ -3583,7 +3583,7 @@ test:track` 43 tracking invariants · headless QA harnesses under
 
 ### Interface localization (October 2026)
 
-Shipped locales: **en** (source and fallback), **es**, **ru** and **zh-CN**. The code lives in
+Shipped locales: **en** (source and fallback), **es**, **ru**, **zh-CN** and **zh-TW**. The code lives in
 `src/i18n/`. How to mark UI text: [`docs/I18N.md`](./I18N.md); translator workflow:
 [`docs/TRANSLATORS.md`](./TRANSLATORS.md).
 
@@ -3594,8 +3594,8 @@ Shipped locales: **en** (source and fallback), **es**, **ru** and **zh-CN**. The
   value). English has no catalog; every other locale is
   `src/i18n/locales/<code>.json`, and a missing entry falls back to English.
 - **Locale choice.** `?lang=` → stored choice (`gev:locale:v1`) →
-  `navigator.languages` → English. Chinese tags map to `zh-CN`; tags containing
-  `Hant`, `TW`, `HK` or `MO` are skipped. `?lang=` is not stored, and share links drop
+  `navigator.languages` → English. Chinese tags default to `zh-CN`; `Hant`, `TW`, `HK` and `MO`
+  select `zh-TW`. Explicit `Hans` selects `zh-CN` regardless of region. `?lang=` is not stored, and share links drop
   it, so a recipient keeps their own language. The globe picker in the
   bottom-right corner (`src/ui/localePicker.js`) stores the choice, drops
   `?lang=` (keeps it only when storage is blocked) and reloads. It asks first