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/hudLocalityFormat.js

@@ -35,7 +35,16 @@ export const LOCALIZED_LOCALITY = Object.freeze({
     }),
   sector: ({ latDeg, lonDeg }) =>
     t('SECTOR {lat} {lon}', {
-      lat: hudCoordinate(latDeg, t('N'), t('S')),
-      lon: hudCoordinate(lonDeg, t('E'), t('W')),
+      // Hemisphere letters, not compass words: 21.33N, never 21.33北.
+      lat: hudCoordinate(
+        latDeg,
+        t('N', { context: 'hemisphere' }),
+        t('S', { context: 'hemisphere' }),
+      ),
+      lon: hudCoordinate(
+        lonDeg,
+        t('E', { context: 'hemisphere' }),
+        t('W', { context: 'hemisphere' }),
+      ),
     }),
 });