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>

libs/shared/docs/behaviour-spec.mdx

@@ -20,7 +20,7 @@ tested where._
 
 Every bullet below is a real test name from the suite — an `it()` title (frontend) or a test
 method name (backend), read as a sentence. Nothing here is hand-written prose: this page
-**is** the suite, reshaped for a business reader. 451 frontend behaviours across
+**is** the suite, reshaped for a business reader. 456 frontend behaviours across
 9 contexts; 236 backend behaviours across 41 test
 classes.
 
@@ -406,7 +406,15 @@ classes.
 
 - loads and parses the cross-owner list
 - deletes optimistically and confirms via the admin endpoint
-- rolls back the removal when the delete fails
+- rolls back the removal and surfaces the error when the delete fails
+- clears a stale error on the next delete attempt
+
+#### ApplicationsStore
+
+- loads and parses the list
+- cancels optimistically and confirms via the DELETE endpoint
+- rolls back the removal and surfaces the error when the cancel fails
+- clears a stale error on the next cancel attempt
 
 #### STEPS (fixed)