feat(desktop): add German (de) locale to the desktop UI

apps/desktop/DESIGN.md

@@ -284,9 +284,9 @@ long transcript or a busy terminal.
 
 - Every user-facing string goes through `useI18n()` (`src/i18n/context.tsx`).
   No literals in JSX.
-- **Update all locales together** — `en`, `ja`, `zh`, `zh-hant`. A string change
+- **Update all locales together** — `en`, `ja`, `zh`, `zh-hant`, `de`. A string change
   in `en.ts` that skips the others is a regression (drifted punctuation,
-  stale labels). Keep trailing-punctuation and tone consistent across all four.
+  stale labels). Keep trailing-punctuation and tone consistent across all five.
 
 ## State (TypeScript)
 
@@ -331,7 +331,7 @@ The detailed state contract lives in the scoped
 - [ ] Hot interactions avoid broad subscriptions, layout thrash, and
       `transition-all`?
 - [ ] Keyboard ownership and single-action `Esc` behavior are correct?
-- [ ] All four locales updated for any new/changed string?
+- [ ] All five locales updated for any new/changed string?
 - [ ] `cursor-pointer`, focus ring, and `Esc`-to-close behave?
 - [ ] Touched a primitive, token, or variant? Its named-contract entry in this
       file is updated in the same change.

apps/desktop/src/i18n/catalog.ts

@@ -1,4 +1,5 @@
 import { ar } from './ar'
+import { de } from './de'
 import { en } from './en'
 import { ja } from './ja'
 import type { Locale, Translations } from './types'
@@ -10,5 +11,6 @@ export const TRANSLATIONS: Record<Locale, Translations> = {
   zh,
   'zh-hant': zhHant,
   ja,
-  ar
+  ar,
+  de
 }

apps/desktop/src/i18n/context.test.tsx

@@ -135,7 +135,9 @@ describe('I18nProvider', () => {
 
   it('does not overwrite unsupported configured languages', async () => {
     const configClient: I18nConfigClient = {
-      getConfig: vi.fn().mockResolvedValue({ display: { language: 'de' } }),
+      // Intentionally not a real locale code (unlike 'de', 'es', etc. which may
+      // become supported over time) — guarantees this stays unsupported.
+      getConfig: vi.fn().mockResolvedValue({ display: { language: 'xx' } }),
       saveConfig: vi.fn()
     }