Smart review: name the source deck on every card, break the counts down

"It contains mistakes I either did more than once or from decks that I've
removed - there's no way there's 34 to repeat with 2 decks." The counts were
honest (dueItems/mistakeItems only resolve live deck+activity pairs; a seeded
repro with 37 SRS rows and 30 live shows "30 due"), but nothing in the UI said
where an item came from: a deck created twice from the same notes shows every
question twice, and a deck that survived a delete or a sync looks like a ghost.
Both read as a broken list.

- SessionFeed gains showDeck (review + mistakes): a deck chip above every
  card, so two identical questions are visibly two decks.
- The Smart review card breaks its numbers down under the buttons:
  "Due by deck: Sky deck - 17 - Sky deck (2) - 17", capped at three decks
  plus "+n more" (groupByDeck in lib/learning).
- New i18n keys (en + cs, plural forms for "+n more").

review-test pins it: a twin-deck seed renders the breakdown and both deck
chips in the feed. Version 0.6.8, SW v90.

components/LibraryScreen.tsx

@@ -16,7 +16,7 @@ import { createFolder, folderStore, renameFolder } from "@/lib/folders";
 import { deleteHomework, homeworkStore, sortHomework } from "@/lib/homework";
 import { learningStore } from "@/lib/history";
 import { useI18n } from "@/lib/i18n";
-import { dueItems, mistakeItems, todayAnswerCount } from "@/lib/learning";
+import { dueItems, groupByDeck, mistakeItems, todayAnswerCount, type ReviewItem } from "@/lib/learning";
 import { isReported, reportStore } from "@/lib/question-reports";
 import { settingsStore } from "@/lib/settings";
 import { ALL_FOLDERS, folderSelection, useSelectedFolder } from "@/lib/nav";
@@ -120,20 +120,22 @@ export function LibraryScreen({ initialTab = "decks" }: { initialTab?: "decks" |
      deleting a deck or reporting a question must drop its due schedules and
      its mistakes right away, matching ReviewClient. */
   const deckIds = useMemo(() => new Set(decks.map((deck) => deck.id)), [decks]);
-  const due = useMemo(
+  const dueItemsList = useMemo(
     () => {
       void reports;
-      return dueItems(srs).filter((item) => deckIds.has(item.deck.id) && !isReported(item.deck.id, item.activity.id)).length;
+      return dueItems(srs).filter((item) => deckIds.has(item.deck.id) && !isReported(item.deck.id, item.activity.id));
     },
     [srs, deckIds, reports],
   );
-  const mistakes = useMemo(
+  const mistakeItemsList = useMemo(
     () => {
       void reports;
-      return mistakeItems(answers).filter((item) => deckIds.has(item.deck.id) && !isReported(item.deck.id, item.activity.id)).length;
+      return mistakeItems(answers).filter((item) => deckIds.has(item.deck.id) && !isReported(item.deck.id, item.activity.id));
     },
     [answers, deckIds, reports],
   );
+  const due = dueItemsList.length;
+  const mistakes = mistakeItemsList.length;
   const todayCount = useMemo(() => todayAnswerCount(answers), [answers]);
   const importRef = useRef<HTMLInputElement>(null);
 
@@ -370,6 +372,19 @@ export function LibraryScreen({ initialTab = "decks" }: { initialTab?: "decks" |
             <LinkLoadingScreen messageKey="review.preparing" />
           </Link>
         </div>
+        {/* Where the numbers come from. "34 due" over a two-deck library read
+            as duplicated or ghost items; naming the decks (and how many each
+            contributes) makes the count verifiable at a glance. */}
+        {due > 0 ? (
+          <p className="text-[11.5px] leading-snug text-[var(--text-muted)]">
+            {t("review.dueByDeck", { list: deckBreakdown(dueItemsList, t) })}
+          </p>
+        ) : null}
+        {mistakes > 0 ? (
+          <p className="text-[11.5px] leading-snug text-[var(--text-muted)]">
+            {t("review.mistakesByDeck", { list: deckBreakdown(mistakeItemsList, t) })}
+          </p>
+        ) : null}
       </section>
 
       {/* gentle backup nudge — dismissible, never blocks ---------------- */}
@@ -664,3 +679,15 @@ export function LibraryScreen({ initialTab = "decks" }: { initialTab?: "decks" |
     </div>
   );
 }
+
+/** "Sky deck · 17 · Biology · 14 (+2 more)" — capped so the line stays short. */
+function deckBreakdown(
+  items: ReviewItem[],
+  t: (key: string, vars?: Record<string, string | number>) => string,
+): string {
+  const groups = groupByDeck(items);
+  const shown = groups.slice(0, 3).map((group) => `${group.label} · ${group.count}`);
+  const rest = groups.length - shown.length;
+  if (rest > 0) shown.push(t("review.moreDecks", { n: rest }));
+  return shown.join(" · ");
+}