fix(ssp): route cancel/delete through runSubmit, surface the error (RB-20)

ApplicationsStore.cancel and AdminCasesStore.delete rolled an optimistic
write back on failure but showed no message — a bare catch with no
Result and no error channel (CQ-002). Both now call runSubmit and set a
lastError signal on failure, mirroring createSubmitChangeRequest in the
same folder. Each page renders the error with the existing app-alert
atom, the same pattern brief.page.ts already uses for lastError.

Added a spec file for ApplicationsStore (none existed) and extended
AdminCasesStore's spec, each asserting the rollback AND the surfaced
error. Verified both new assertions fail without the fix (an Edit
undo/redo of the store method, not git checkout).

Regenerated libs/shared/docs/behaviour-spec.mdx (gen:behaviour-spec) to
pick up the new/renamed test names. Marked RB-20 done in 99-backlog.md
and recorded the change in implementation/rb-20.md.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

apps/ssp/src/app/registratie/application/applications.store.ts

@@ -1,5 +1,6 @@
 import { Injectable, inject, signal } from '@angular/core';
 import { RemoteData } from '@shared/application/remote-data';
+import { runSubmit, SUBMIT_FAILED } from '@shared/application/submit';
 import { Aanvraag } from '@registratie/domain/aanvraag';
 import {
   ApplicationsAdapter,
@@ -15,7 +16,8 @@ type Err = Error | undefined;
  * the row SYNCHRONOUSLY, so the block disappears deterministically — no dependence on
  * change-detection timing, HTTP caching, or a resource `reload()`. `reload()` re-fetches
  * so a page revisit reflects auto-approval (Concept → In behandeling → Goedgekeurd is
- * computed server-side on read).
+ * computed server-side on read). Cancel goes through `runSubmit` and rolls back plus
+ * surfaces `lastError` on failure (RB-20).
  */
 @Injectable({ providedIn: 'root' })
 export class ApplicationsStore {
@@ -24,6 +26,11 @@ export class ApplicationsStore {
   private state = signal<RemoteData<Err, Aanvraag[]>>({ tag: 'Loading' });
   readonly applications = this.state.asReadonly();
 
+  /** Set on a failed cancel (RB-20): the optimistic removal already rolled back by
+      then, this is only the message for the alert the page renders above the list. */
+  private error = signal<string | null>(null);
+  readonly lastError = this.error.asReadonly();
+
   constructor() {
     void this.load();
   }
@@ -50,16 +57,19 @@ export class ApplicationsStore {
   }
 
   /** Cancel a Concept: drop it now (synchronous, guaranteed), then confirm the DELETE.
-      No resync — the delete succeeded, so the optimistic removal is authoritative. */
+      No resync — the delete succeeded, so the optimistic removal is authoritative. On
+      failure, roll back AND surface the error (RB-20) — a silent reappearance leaves the
+      user guessing why the block came back. */
   async cancel(id: string) {
     const before = this.state();
     if (before.tag === 'Success') {
       this.state.set({ tag: 'Success', value: before.value.filter((a) => a.id !== id) });
     }
-    try {
-      await this.adapter.cancel(id);
-    } catch {
+    this.error.set(null);
+    const r = await runSubmit(() => this.adapter.cancel(id), SUBMIT_FAILED);
+    if (!r.ok) {
       this.state.set(before); // roll back: the block reappears
+      this.error.set(r.error);
     }
   }
 }