Add localization support with German translations

[Peaj]

Summary

Add application localization so DeskGrid can support German (de) in addition to the current English UI.

Problem

All visible UI text is currently hard-coded in English. This blocks German-language classroom use and makes future language support harder.

Scope

• Introduce a localization/i18n layer for user-facing UI strings.
• Add German translations for the current interface.
• Provide a way to select or detect the active language.
• Ensure exported/imported project data remains language-neutral.

Suggested implementation

• Centralize user-facing strings instead of hard-coding them in components.
• Add a translation resource structure for at least en and de.
• Default to English if a translation key is missing.
• Keep internal IDs, schema fields, and persisted data unchanged.

Acceptance criteria

• The app can render in German.
• Core UI areas are translated: top bar, toolbar, solver panel, roster/bench, layout sidebar, constraint panel, notices, and import/export messages.
• Switching languages does not affect saved project/roster/layout compatibility.
• New strings are added through the localization system rather than inline literals.

Notes

This should be implemented in a way that makes adding more languages later straightforward.