i18n (2/4): translation layer and German (Du) + English for the app shell and event dialog #626

Split from #355 (the whole issue does not fit one run). Common rules: tests with teeth, Playwright for UI behaviour (CLAUDE.md §3.3). Platform owner preferences (freeshard#168): use them as defaults if the platform offers them; otherwise default from the browser (navigator.language, Intl time zone) and note the fallback in CONCEPTS.md — do not block on the platform. Builds on #625.

Acceptance criteria

• A small i18n layer (no framework): message catalogues per locale, a t(key, params) with plural support, locale chosen from the preference of Preferences (1/4): week start, 12/24 h, default duration/calendar/reminder, time zone; Intl formatting; all-day off-by-one #625.
• Locales: Deutsch (Du) as today's wording, English. Strings of public/index.html, app.js (shell, sidebar, settings, toasts) and eventModal.js are extracted; the other modules follow in #(3/4).
• Playwright: switching the language re-renders the shell and the event dialog in English without reload; German stays the default.