Adds a hardcoded changelog (internal/changelog) served via
GET /api/changelog and /api/changelog/unseen-count?since=<iso>, a
/changelog page that renders entries newest-first, and a sidebar
"Neuigkeiten" link with a lime badge showing the count of unseen
entries since the caller's last visit (localStorage stamp).
- internal/changelog: Entry struct, 11 pre-populated entries covering
everything shipped so far (Dashboard, Projects/Deadlines/Appointments,
CalDAV, Checklists v2, Glossary, Courts, Invitations, Settings,
Paliad rename, and the changelog itself).
- Handler: public via auth-gated protected mux. Lexicographic string
compare treats YYYY-MM-DD entries and ISO 8601 cutoffs symmetrically.
- Sidebar: new sidebar-changelog link before the Einladen button; the
badge is populated by a fetch on every page load, suppressed on
/changelog itself to avoid flash, and cleared on visit by stamping
localStorage in changelog.ts's DOMContentLoaded handler.
- i18n: DE + EN keys for nav, page chrome, and tag labels.
- Unit tests for sort order, copy semantics, and same-day cutoff.
Task: t-paliad-027
94e2fc0024