feat(t-paliad-086): trigger-event Fristenrechner mode + working_days primitive — PR-2

Adds the second Fristenrechner mode (mirrored from youpc.org's deadline
calc): pick a UPC trigger event + date, see all deadlines that flow
from it. Coexists with the existing course-of-proceedings timeline mode
via a tab toggle on /tools/fristenrechner.

Backend:
- internal/services/event_deadline_service.go — EventDeadlineService.
  ListTriggerEvents (alphabetical), Calculate (resolves all deadlines
  flowing from a trigger). Routes through HolidayService for weekend +
  holiday rollover. Honours the new working_days unit. Resolves
  composite rules (alt_* + combine_op) by computing both legs and
  picking max/min. Used by R.198/R.213 ("31d OR 20wd, whichever is
  longer") imported in PR-1.
- internal/services/event_deadline_service_test.go — covers
  addWorkingDays (forward, backward, zero, holiday-skip), composite
  rule semantics, before-timing.
- internal/handlers/fristenrechner.go — two new endpoints:
  GET /api/tools/trigger-events, POST /api/tools/event-deadlines.
- handlers.Services / dbServices: new EventDeadline / eventDeadline
  field; wired in cmd/server/main.go from the same HolidayService.

Frontend:
- frontend/src/fristenrechner.tsx — tab strip + second wizard panel
  (3 steps: trigger picker → date → flat result list).
- frontend/src/client/fristenrechner.ts — initEventMode wiring,
  typeahead filter over the 102 trigger events, Calculate flow,
  bilingual rendering, composite-rule labels, lang-change refresh.
- frontend/src/client/i18n.ts — 27 new keys (DE+EN) under
  deadlines.mode.* and deadlines.event.* (incl. units, timing).
- frontend/src/styles/global.css — fristen-mode-tabs, mode-panel,
  event-list, event-result-row visual style.

Working-day arithmetic detail: the new addWorkingDays helper steps
one day at a time and skips runs of non-working days (Sat/Sun + DE
federal + UPC vacations seeded via paliad.holidays). Day-zero is the
caller's job — addWorkingDays(0) returns the input unchanged so
callers can decide whether to roll forward via AdjustForNonWorkingDays.

Composite-rule resolution: when a row carries alt_duration_value +
alt_duration_unit + combine_op, Calculate computes both legs,
picks max/min, and surfaces a compositeNote like
"max(31 days, 20 working_days) → working_days leg" so the UI can
explain which leg won.

PR-3 will land Tier 1 bug fixes from the audit (CCR adaptive,
UPC_APP grounds anchoring, EP_GRANT priority, rule-code normalisation).

cmd/server/main.go

@@ -130,6 +130,7 @@ func main() {
 			Calculator:     services.NewDeadlineCalculator(holidays),
 			Users:          users,
 			Fristenrechner: services.NewFristenrechnerService(rules, holidays),
+			EventDeadline:  services.NewEventDeadlineService(pool, services.NewDeadlineCalculator(holidays), holidays),
 			Dashboard:      services.NewDashboardService(pool, users),
 			Note:           services.NewNoteService(pool, projectSvc, appointmentSvc),
 			ChecklistInst:  services.NewChecklistInstanceService(pool, projectSvc),