feat(10-04): search, link and unlink related records through the relation manager

- relation-manager registered in the field registry; renders only on an
  existing record, never on create, and is never part of the save body
- RelationManager: relation schema label and comment, debounced search,
  selectable linked list (DataTable relation variant), toolbar buttons in
  declared order, confirmed unlink with plural messages and toasts
- RelationPickerModal: Reka Dialog (aria-modal, focus trap, Esc) over the
  candidates endpoint five per page, selection kept across pages, Dodaj (N)
  POSTs link, focus returns to the opener
- admin OpenAPI documents search, sort, dir, page and per_page on the linked
  and candidate relation routes so the SPA sends them typed
- neutral acme.demo.widgets members fixtures and relation smoke tests

admin/src/api/schema.d.ts

@@ -1454,7 +1454,18 @@ export interface paths {
         /** List linked relation records */
         get: {
             parameters: {
-                query?: never;
+                query?: {
+                    /** @description Search term over the panel's searchable columns */
+                    search?: string;
+                    /** @description Sort column (a sortable panel column) */
+                    sort?: string;
+                    /** @description Sort direction (asc or desc) */
+                    dir?: string;
+                    /** @description Page */
+                    page?: number;
+                    /** @description Records per page (1-100, default 20) */
+                    per_page?: number;
+                };
                 header?: never;
                 path: {
                     /** @description Vendor */
@@ -1537,7 +1548,18 @@ export interface paths {
         /** List relation candidates */
         get: {
             parameters: {
-                query?: never;
+                query?: {
+                    /** @description Search term over the panel's searchable columns */
+                    search?: string;
+                    /** @description Sort column (a sortable panel column) */
+                    sort?: string;
+                    /** @description Sort direction (asc or desc) */
+                    dir?: string;
+                    /** @description Page */
+                    page?: number;
+                    /** @description Records per page (1-100, default 20) */
+                    per_page?: number;
+                };
                 header?: never;
                 path: {
                     /** @description Vendor */