Mobile Media table: stacked cards + mobile pagination (Slice 3)

Below md, the Media DataTable renders as MobileCardRow cards: title as
primary, plus size/HDR/library/year (3-5 fields, null-safe). Card tap
navigates to /files?path=... (same handleRowClick as desktop). The TanStack
column-visibility toggle is absent below md (the card picks the fields).

Pagination is preserved via a standalone MediaMobilePagination component
that mirrors DataTablePagination semantics (rows count, page-size select,
page indicator, prev/next with correct disabled states) off the raw
PaginationState. The duplication is flagged tech debt -- extracting a shared
TablePagination is a follow-up, out of scope for this slice.

Desktop (md+) is byte-for-byte identical: the isMobile===false branch
renders the same DataTable with the same props. enableRowSelection state is
vestigial (no batch consumer on either path); navigation is the correct
primary mobile interaction.

Tests: 5 new covering mobile cards + hidden column toggle + pagination +
card-tap navigation, and desktop DataTable + column toggle. matchMedia
mocked per-breakpoint. 94 tests pass; lint/build green.

Refs openspec/changes/mobile-responsive-parity/ (spec R3, tasks slice 3).

frontend/src/pages/__tests__/Media.test.tsx

@@ -129,7 +129,11 @@ vi.mock("../../hooks/useDashboard", () => ({
 
 // usePersistentState reads/writes localStorage; clear between tests so the
 // offset/pageSize/columnVisibility state never leaks across cases.
+// matchMedia must be stubbed so useIsMobile (md:768px) and usePrefersSmallScreen
+// (900px) resolve without TypeError in jsdom. Default to desktop (matches:false)
+// so the DataTable path renders by default; mobile tests override.
 beforeEach(() => {
+	setMatchMedia(false);
 	window.localStorage.clear();
 	navigate.mockClear();
 	status = statusFixture();
@@ -152,6 +156,30 @@ beforeEach(() => {
 	};
 });
 
+/** Stub window.matchMedia so useIsMobile / usePrefersSmallScreen resolve in jsdom. */
+function setMatchMedia(matches: boolean) {
+	const listeners: ((e: MediaQueryListEvent) => void)[] = [];
+	window.matchMedia = vi.fn().mockImplementation((query: string) => ({
+		matches: query.includes("768") ? matches : false,
+		media: query,
+		onchange: null,
+		addEventListener: (
+			_evt: string,
+			listener: (e: MediaQueryListEvent) => void,
+		) => listeners.push(listener),
+		removeEventListener: (
+			_evt: string,
+			listener: (e: MediaQueryListEvent) => void,
+		) => {
+			const idx = listeners.indexOf(listener);
+			if (idx >= 0) listeners.splice(idx, 1);
+		},
+		addListener: () => {},
+		removeListener: () => {},
+		dispatchEvent: () => false,
+	}));
+}
+
 describe("Media (slice 7b — TanStack DataTable + server-driven pagination)", () => {
 	it("exposes exactly the 15 locked toggleable columns", async () => {
 		render(<Media />);
@@ -261,3 +289,68 @@ describe("Media (slice 7b — TanStack DataTable + server-driven pagination)", (
 		).toBeInTheDocument();
 	});
 });
+
+describe("Media (mobile card layout — slice 3)", () => {
+	it("renders cards with the title as primary below md", () => {
+		setMatchMedia(true);
+		render(<Media />);
+
+		// Card titles render (primary field).
+		expect(screen.getByText("Inception")).toBeInTheDocument();
+		expect(screen.getByText("Matrix")).toBeInTheDocument();
+
+		// Card field labels render (at least once per row).
+		expect(screen.getAllByText("Size").length).toBeGreaterThanOrEqual(2);
+		expect(screen.getAllByText("HDR").length).toBeGreaterThanOrEqual(2);
+		expect(screen.getAllByText("Library").length).toBeGreaterThanOrEqual(2);
+		expect(screen.getAllByText("Year").length).toBeGreaterThanOrEqual(2);
+
+		// Desktop table headers do NOT render on mobile.
+		expect(screen.queryByRole("columnheader", { name: "Title" })).toBeNull();
+		expect(screen.queryByRole("columnheader", { name: "Bitrate" })).toBeNull();
+	});
+
+	it("hides the column-visibility toggle below md", () => {
+		setMatchMedia(true);
+		render(<Media />);
+
+		expect(screen.queryByRole("button", { name: /Columns/ })).toBeNull();
+	});
+
+	it("renders pagination controls below the cards on mobile", () => {
+		setMatchMedia(true);
+		render(<Media />);
+
+		expect(screen.getByText("2 rows")).toBeInTheDocument();
+		expect(screen.getAllByText(/Page 1 of 1/).length).toBeGreaterThan(0);
+		expect(
+			screen.getByRole("button", { name: "Previous page" }),
+		).toBeDisabled();
+		expect(
+			screen.getByRole("button", { name: "Next page" }),
+		).toBeInTheDocument();
+	});
+
+	it("navigates to the file browser when a card is tapped on mobile", async () => {
+		setMatchMedia(true);
+		render(<Media />);
+
+		await userEvent.click(screen.getByText("Inception"));
+
+		expect(navigate).toHaveBeenCalledTimes(1);
+		expect(navigate).toHaveBeenCalledWith(
+			`/files?path=${encodeURIComponent("/media/movies/Inception.mkv")}`,
+		);
+	});
+
+	it("renders the DataTable (not cards) at desktop width", () => {
+		render(<Media />);
+
+		// Desktop column headers render.
+		expect(
+			screen.getByRole("columnheader", { name: "Title" }),
+		).toBeInTheDocument();
+		// Column-visibility toggle is present.
+		expect(screen.getByRole("button", { name: /Columns/ })).toBeInTheDocument();
+	});
+});