Task e57facc2: Undo and live SSE updates in the admin PWA

- api/events.ts: fetch-based reader for GET /api/v1/admin/events (admin JWT
  in the Authorization header via apiFetch, never in the URL), an incremental
  SSE frame parser that buffers frames split across chunks and drops malformed
  frames, and bounded-backoff reconnect with a clean abort on stop.
- api/useTransitions.tsx: one stream per signed-in shell (TransitionsProvider
  in App) fanned out to screens; no provider means no stream.
- Today: Undo posts /instances/{id}/uncomplete, disables every Undo while in
  flight, shows a dismissible message on failure, and refetches the snapshot;
  any transition refetches in place. Mark done stays inert.
- History: quest transitions refetch the active filter in place; rows stay
  read-only.
- Tests for parsing, stream auth/teardown/reconnect, Undo, and live refetch.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

admin/src/today/TodayTab.css

@@ -241,6 +241,37 @@
   color: var(--nq-brand-purple);
 }
 
+.today-action:disabled {
+  cursor: default;
+  opacity: 0.5;
+}
+
+/* Non-blocking Undo failure message. */
+.today-action-error {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 10px;
+  padding: var(--nq-a-density-row-pad);
+  border-radius: var(--nq-a-density-radius);
+  background: var(--nq-a-danger-bg);
+  color: var(--nq-a-danger-strong);
+  font-size: 13px;
+  font-weight: 600;
+}
+
+.today-action-dismiss {
+  flex: none;
+  padding: 4px 8px;
+  border: none;
+  background: none;
+  color: inherit;
+  font-family: inherit;
+  font-size: 12.5px;
+  font-weight: 700;
+  cursor: pointer;
+}
+
 .today-empty {
   padding: var(--nq-a-density-card-pad);
   font-size: 13px;