feat(portal-frontend): add worklist and case-detail typed interfaces and API services

Types mirror New.Api.Contracts verbatim; all write calls use the href
given in a case's actions block rather than a client-built URL.

portal-frontend/src/app/shared/api-error.types.ts

@@ -0,0 +1,25 @@
+// Mirrors New.Api.Contracts error shapes verbatim.
+
+export interface FieldError {
+  field: string;
+  message: string;
+  detail?: string | null;
+}
+
+/** 400 - legacy write-through validation failures only (seam B). */
+export interface ErrorsResponse {
+  errors: FieldError[];
+}
+
+/** 422 - a single named domain-invariant failure. Render `invariant` and
+ * `message` as given; the set of invariants isn't fixed client-side
+ * knowledge (ADR-003), so no switch/enum maps over it. */
+export interface InvariantViolationResponse {
+  invariant: string;
+  message: string;
+}
+
+/** 409 - a conflict, e.g. release-ownership blocked by prior domain writes. */
+export interface MessageResponse {
+  message: string;
+}