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>

internal/server/suggestions.go

@@ -10,10 +10,6 @@ import (
 	"code.cubecraftlabs.com/Maniacs_Mansion/Meal-Quest/internal/suggest"
 )
 
-type deck struct {
-	Suggestions []suggest.Suggestion `json:"suggestions"`
-}
-
 // postSuggestionBatch starts a manual suggestion batch and answers 202 with
 // its batch_id and the job_id that GET /jobs/{id} reports on, without
 // waiting for the job. When a batch is already pending or running it
@@ -30,15 +26,15 @@ func postSuggestionBatch(logger *slog.Logger, svc *suggest.Service) http.Handler
 }
 
 // getSuggestionDeck answers GET /suggestions/deck with the pending
-// suggestions, oldest first.
+// suggestions, oldest first, and the batch pending or running, or null.
 func getSuggestionDeck(logger *slog.Logger, svc *suggest.Service) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
-		list, err := svc.Deck(r.Context())
+		d, err := svc.Deck(r.Context())
 		if err != nil {
 			writeSuggestionError(w, r, logger, "list suggestion deck", err)
 			return
 		}
-		writeJSON(w, r, logger, http.StatusOK, deck{Suggestions: list})
+		writeJSON(w, r, logger, http.StatusOK, d)
 	}
 }