[Ohualtex]

Add German (de) interface localization across macOS, Windows, and Linux, including the Deutsch language picker entry and system-locale resolution.

• Translate all 371 macOS catalog entries and 296 Windows/Linux-only entries, preserving placeholders and supplying German singular/plural forms.
• Register German in the macOS picker and both app targets, the frontend, the string generator, and Rust.
• Regenerate the shared table, including 38 upstream catalog keys missing from the previously committed generated file. Remove the duplicate Permissions extra entry; its identical translations remain in the shared catalog.
• Add coverage for regional locale resolution, fallback, interpolation, dates, plural forms, Apple format specifiers, and cross-platform language registration.

Validation: 366 frontend tests and all 5 Rust i18n tests pass; TypeScript checking, generator freshness checking, and the Vite production build pass. Existing non-German translations were checked for preservation. The contributor manually checked the German browser preview and reported no visible issues. Native macOS build and UI validation were not performed on this Windows machine.

Closes #436.
Related to the German-language request in #410; this PR is independent of the Turkish localization PR #414.

[Ohualtex]

Related contribution: #414 adds Turkish localization independently of this German localization PR. When merging both contributions, the shared language registries will need to retain both tr and de.

[Ohualtex]

Superseded by #701. I rebuilt the German contribution directly on the updated main used for validation, completed the newly added strings and kept the change focused on 11 localization, registration and test files in one commit. Closing this PR in favor of #701; the new PR references this discussion to preserve the earlier context.