i18n(zh-CN): complete and ship Simplified Chinese

- Translate the remaining 910 strings and review the whole catalog (meaning,
  terms, punctuation, label length) against the screens.
- Register zh-CN: any zh tag maps to it except Traditional ones (Hant, TW,
  HK, MO), which keep English until a Traditional catalog ships.
- Hemisphere letters get their own context so the HUD sector line reads
  21.33N, not a compass word; the Bhote Koshi imagery dates follow
  year-first languages (2021年10月16日).
- cjk.css: tight tracking for Chinese text and a 9px floor on the dock voice
  labels and layer buttons; Latin HUD readouts keep their look.
- docs/translations/zh-CN.md: style, plurals and glossary.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/ui/localePicker.test.mjs

@@ -40,7 +40,7 @@ test('the current language comes first, the rest by their own names', () => {
         nativeNames: LOCALE_NAMES,
       }),
     ),
-    ['ru', 'en', 'es'],
+    ['ru', 'en', 'es', 'zh-CN'],
   );
   assert.deepEqual(
     codes(
@@ -50,7 +50,7 @@ test('the current language comes first, the rest by their own names', () => {
         nativeNames: LOCALE_NAMES,
       }),
     ),
-    ['es', 'en', 'ru'],
+    ['es', 'en', 'ru', 'zh-CN'],
   );
   assert.deepEqual(
     codes(
@@ -60,7 +60,7 @@ test('the current language comes first, the rest by their own names', () => {
         nativeNames: LOCALE_NAMES,
       }),
     ),
-    ['en', 'es', 'ru'],
+    ['en', 'es', 'ru', 'zh-CN'],
   );
   const rows = buildLocaleOptions({
     locales: MANY,
@@ -263,7 +263,7 @@ test('keyboard model: open on the current language, move, choose, close', () =>
   assert.deepEqual(model.keydown({ key: 'ArrowDown' }), { handled: true });
   assert.equal(model.activeOption.code, 'en');
   model.keydown({ key: 'End' });
-  assert.equal(model.activeOption.code, 'es');
+  assert.equal(model.activeOption.code, 'zh-CN');
   model.keydown({ key: 'Home' });
   assert.equal(model.activeOption.code, 'ru');
   model.keydown({ key: 'ArrowUp' });