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/SessionFeed.tsx

@@ -4,7 +4,7 @@ import { useCallback, useEffect, useRef, useState } from "react";
 
 import { ActivityRenderer } from "@/components/activities";
 import { AiDisclaimer } from "@/components/AiDisclaimer";
-import { XIcon } from "@/components/icons";
+import { LayersIcon, XIcon } from "@/components/icons";
 import { ReportQuestionButton } from "@/components/ReportQuestionButton";
 import { ProgressBar } from "@/components/ui";
 import type { Activity, Deck } from "@/lib/schemas";
@@ -32,6 +32,7 @@ export function SessionFeed({
   items,
   timerSeconds,
   noHints = false,
+  showDeck = false,
   testId,
   onExit,
   onAnswered,
@@ -41,6 +42,9 @@ export function SessionFeed({
   items: SessionItem[];
   timerSeconds?: number;
   noHints?: boolean;
+  /** Review/mistakes: show which deck each card comes from. Two decks with
+   *  the same question otherwise read as one item repeating itself. */
+  showDeck?: boolean;
   testId?: string;
   onExit: () => void;
   onAnswered?: (item: SessionItem, correct: boolean, meta?: { userAnswer?: string }) => void;
@@ -203,6 +207,14 @@ export function SessionFeed({
             className="flex h-full snap-start snap-always items-stretch justify-center overflow-y-auto overscroll-y-contain"
           >
             <div className="mx-auto flex min-h-full w-full max-w-xl flex-col gap-4 px-4 pb-24 pt-24 [justify-content:safe_center] [&>*]:shrink-0">
+              {showDeck ? (
+                <div className="flex flex-wrap items-center gap-2">
+                  <span className="glass-chip inline-flex max-w-full items-center gap-1.5 rounded-full px-3 py-1 text-xs font-semibold text-frost-600 dark:text-frost-300">
+                    <LayersIcon className="size-3.5 shrink-0" />
+                    <span className="truncate">{item.deck.title}</span>
+                  </span>
+                </div>
+              ) : null}
               <ActivityRenderer
                 activity={item.activity}
                 active={index === itemIndex}