feat: add deadline management frontend (Phase 1G)

- Fristen page with list view (sortable, filterable by status/case)
- Calendar view with month navigation and deadline dots
- Deadline calculator page (proceeding type + trigger date = timeline)
- Traffic light urgency: red (overdue), amber (this week), green (OK)
- Backend: GET /api/deadlines (all tenant deadlines), GET /api/proceeding-types
- API client: added patch() method
- Types: DeadlineRule, ProceedingType, CalculatedDeadline, RuleTreeNode

backend/internal/handlers/deadline_rules.go

@@ -39,6 +39,17 @@ func (h *DeadlineRuleHandlers) List(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, http.StatusOK, rules)
 }
 
+// ListProceedingTypes handles GET /api/proceeding-types
+func (h *DeadlineRuleHandlers) ListProceedingTypes(w http.ResponseWriter, r *http.Request) {
+	types, err := h.rules.ListProceedingTypes()
+	if err != nil {
+		writeError(w, http.StatusInternalServerError, "failed to list proceeding types")
+		return
+	}
+
+	writeJSON(w, http.StatusOK, types)
+}
+
 // GetRuleTree handles GET /api/deadline-rules/{type}
 // {type} is the proceeding type code (e.g., "INF", "REV")
 func (h *DeadlineRuleHandlers) GetRuleTree(w http.ResponseWriter, r *http.Request) {