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.

lib/learning.ts

@@ -71,6 +71,23 @@ export function mistakeCount(answers: AnswerRecord[], deckId?: string): number {
   return mistakeItems(answers, deckId).length;
 }
 
+/**
+ * Where a review list comes from: one entry per deck, most items first. The
+ * Smart review card shows this under its buttons so a count that mixes decks
+ * ("34 due" with two decks on screen) is self-explanatory instead of looking
+ * like the same mistake twice or an item from a deck that was deleted.
+ */
+export function groupByDeck(items: ReviewItem[]): { label: string; count: number }[] {
+  const groups = new Map<string, { label: string; count: number }>();
+  for (const item of items) {
+    const key = item.deck.id;
+    const group = groups.get(key) ?? { label: item.deck.title, count: 0 };
+    group.count += 1;
+    groups.set(key, group);
+  }
+  return [...groups.values()].sort((a, b) => b.count - a.count || a.label.localeCompare(b.label));
+}
+
 /* ------------------------------------------------------------------ */
 /* Stats                                                               */
 /* ------------------------------------------------------------------ */