feat(cards): display FSRS difficulty level in card browse view

Show per-card difficulty (D: X.X) in the card list alongside existing
metrics (reps, lapses). Hidden for unreviewed cards (difficulty = 0).

https://claude.ai/code/session_019XoFbra6drKrZpHASUzKDq

2 files changed, 23 insertions, 0 deletions

diff --git a/src/client/pages/DeckCardsPage.test.tsx b/src/client/pages/DeckCardsPage.test.tsx
index 16916df..58107d4 100644
--- a/src/client/pages/DeckCardsPage.test.tsx
+++ b/src/client/pages/DeckCardsPage.test.tsx
@@ -327,6 +327,24 @@ describe("DeckCardsPage", () => {
 		expect(screen.getByText("1 lapses")).toBeDefined();
 	});
 
+	it("displays difficulty for reviewed cards", () => {
+		renderWithProviders({
+			initialDeck: mockDeck,
+			initialCards: mockCards,
+		});
+
+		expect(screen.getByText("D: 5.0")).toBeDefined();
+	});
+
+	it("does not show difficulty for new cards", () => {
+		renderWithProviders({
+			initialDeck: mockDeck,
+			initialCards: mockCards,
+		});
+
+		expect(screen.queryByText("D: 0.0")).toBeNull();
+	});
+
 	it("does not show description if deck has none", () => {
 		const deckWithoutDescription = { ...mockDeck, description: null };
 		renderWithProviders({
diff --git a/src/client/pages/DeckCardsPage.tsx b/src/client/pages/DeckCardsPage.tsx
index 2b2ca88..8c839da 100644
--- a/src/client/pages/DeckCardsPage.tsx
+++ b/src/client/pages/DeckCardsPage.tsx
@@ -162,6 +162,11 @@ function NoteGroupCard({
 							{card.lapses > 0 && (
 								<span className="text-muted">{card.lapses} lapses</span>
 							)}
+							{card.difficulty > 0 && (
+								<span className="text-muted">
+									D: {card.difficulty.toFixed(1)}
+								</span>
+							)}
 						</div>
 					))}
 				</div>