Surface replenish failures, follow the active batch, scope deck keys, keep three deck depths

Fixes the Feature 13 review findings on the Discover deck (Maestro task
4fc551f0-3c11-4c15-b638-f2c08a28eb29):

- F13-01: a batch failure (start, poll or job) is shown with its Retry
  whatever the deck has, so a replenish that fails while cards remain is
  no longer silent until the deck runs out.
- F13-02: GET /api/v1/suggestions/deck reports the batch pending or
  running (active_batch, read in the deck's snapshot), and a decision
  that calls for a replenish returns the job of the batch it started or
  joined. The page follows a newly reported active batch, so a reopened
  page or a second session shows skeleton cards while the batch runs
  instead of "All caught up".
- F13-03: ← / → leave the deck alone while it is inert behind a modal
  (the Quest log sheet or its confirmation).
- F13-04: a one- or two-card deck keeps all three depths: skeleton cards
  stand in for the missing ones while a batch finds more Side quests,
  blank stack-edge cards while none is coming.

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

web/src/App.test.tsx

@@ -107,7 +107,7 @@ describe('App routes', () => {
     mockViewport(390)
     stubFetchByPath({
       ...SIGNED_IN,
-      '/api/v1/suggestions/deck': () => jsonResponse(200, { suggestions: [] }),
+      '/api/v1/suggestions/deck': () => jsonResponse(200, { suggestions: [], active_batch: null }),
       '/api/v1/dinners?status=quest_log': () => jsonResponse(200, { dinners: [] }),
     })
     renderAt('/discover')