New section on /settings/account offering PWA installation. On Chromium
it captures beforeinstallprompt and shows a real install button; Safari
(macOS and iOS) gets platform-specific instructions; the section hides
entirely when the app already runs standalone or after installing.
Strings added to both sv.json and en.json.
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
