t-paliad-030. Adds `/agenda` — a single page that merges every visible
deadline and appointment into a day-grouped timeline, the third overview
surface alongside Dashboard and the per-resource lists.
- AgendaService: merges paliad.deadlines + paliad.appointments, gated by
the same team-membership predicate used everywhere else; personal
appointments stay creator-only. Items are sorted by date and tagged
with urgency (overdue / today / tomorrow / this_week / later) so the
client can apply the traffic-light colours without re-deriving buckets.
- GET /api/agenda?from&to&types and GET /agenda with the same server-side
hydration pattern as /dashboard (JSON payload spliced into the shell so
the timeline paints on first frame).
- Frontend: agenda.tsx + client/agenda.ts render a day-grouped timeline
with type/range chips; filters round-trip through the URL.
- Sidebar entry under "Übersicht"; DE+EN i18n across all new keys.
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