i18n(de): complete and ship German

- Translate the remaining 1,868 strings in a neutral infinitive style (no
  Sie/du), with German compounds, decimal commas and CLDR one/other plurals.
- Review the #908 entries: knots are kn everywhere, the avionics codes BRG,
  CRS and DEST stay English like HDG and SPD, expand/collapse is
  ausklappen/einklappen, the radio toggle reads EIN/AUS, density SPARSE is
  DÜNN, the Thermal style is Wärmebild, German quotes in the shot prompt.
- Register de: catalog, LOCALES, LOCALE_NAMES (Deutsch), package boundaries,
  same-as-english (codes, product names, music genres, units).
- Tests: picker order and keyboard model with Deutsch, de/de-AT/de_CH
  normalization, browser de-DE resolves to German.
- docs/translations/de.md: style, plurals, units and glossary; TRANSLATORS.md
  and CURRENT-STATE.md list German.

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

docs/CURRENT-STATE.md

@@ -3733,7 +3733,7 @@ test:track` 43 tracking invariants · headless QA harnesses under
 
 ### Interface localization (October 2026)
 
-Shipped locales: **en** (source and fallback), **es**, **ru**, **zh-CN** and **zh-Hant**. The code lives in
+Shipped locales: **en** (source and fallback), **de**, **es**, **ru**, **zh-CN** and **zh-Hant**. The code lives in
 `src/i18n/`. How to mark UI text: [`docs/I18N.md`](./I18N.md); translator workflow:
 [`docs/TRANSLATORS.md`](./TRANSLATORS.md).