feat(frontend): slice 7a — DataTable wrapper + FileBrowser (TanStack Table)

Web UI rework. Highest-risk slice, part 1 of 2:
- New components/ui/data-table.tsx: generic TanStack Table wrapper on the
  shadcn Table primitive. Controlled rowSelection/columnVisibility/
  pagination, optional selection column (stopPropagation on cell click),
  row-click, column-visibility dropdown, manual-pagination support.
  Hard rule honored: NO getSortedRowModel, NO column resizing/sizing.
- Migrate pages/FileBrowser.impl.tsx off @mui/x-data-grid + @mui/material
  onto DataTable: 5 columns (type/name/ext/size/modified), row-click ->
  ffprobe preview preserved, column-visibility toggle, no pagination.
- DataTable + FileBrowser component tests (RED->GREEN).

Gate: build + lint + test green (22 files / 58 tests).

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

@@ -0,0 +1,127 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import { render, screen } from "@testing-library/react";
+import userEvent from "@testing-library/user-event";
+import { FileBrowser } from "../FileBrowser.impl";
+import type { DirectoryListing, MonitoringMachine } from "../../types";
+
+// usePersistentState (browserState) reads/writes localStorage; clear between tests
+// so the selectedPath / currentDir state never leaks across cases.
+beforeEach(() => {
+	window.localStorage.clear();
+});
+
+function machineFixture(
+	overrides: Partial<MonitoringMachine> = {},
+): MonitoringMachine {
+	return {
+		id: "local",
+		name: "Local",
+		mode: "local",
+		enabled: true,
+		services: ["files", "monitoring"],
+		host: "",
+		port: 22,
+		username: "",
+		key_directory: "",
+		key_name: "",
+		ssh_key_id: "",
+		ssh_private_key_set: false,
+		ssh_private_key_passphrase_set: false,
+		password_set: false,
+		media_root: "",
+		path_prefix: "",
+		jellyfin_url: "",
+		jellyfin_user_id: "",
+		jellyfin_api_key_set: false,
+		jellyseerr_url: "",
+		jellyseerr_api_key_set: false,
+		notes: "",
+		...overrides,
+	};
+}
+
+function listingFixture(
+	entries: {
+		name: string;
+		type: string;
+		size: number;
+		mtime: number;
+	}[],
+): DirectoryListing {
+	return { path: "/", entries, count: entries.length };
+}
+
+let listing: DirectoryListing;
+let machines: MonitoringMachine[];
+
+vi.mock("react-router-dom", () => ({
+	useSearchParams: () => [new URLSearchParams(), vi.fn()],
+	useNavigate: () => vi.fn(),
+}));
+
+vi.mock("../../hooks/useFiles", () => ({
+	useDirectoryListing: () => ({
+		data: listing,
+		isLoading: false,
+		error: null,
+		refetch: vi.fn(),
+	}),
+	useFfprobe: () => ({ data: undefined, isLoading: false, error: null }),
+	useJobTemplates: () => ({ data: [] }),
+	useRunJob: () => ({ isPending: false, mutate: vi.fn(), data: undefined }),
+}));
+
+vi.mock("../../hooks/useSettings", () => ({
+	useMonitoringSettings: () => ({ data: machines }),
+}));
+
+beforeEach(() => {
+	machines = [machineFixture()];
+	listing = listingFixture([
+		{ name: "movies", type: "d", size: 0, mtime: 1_700_000_000 },
+		{ name: "video.mkv", type: "f", size: 1_500_000_000, mtime: 1_700_000_000 },
+		{ name: "notes.txt", type: "f", size: 12, mtime: 1_700_000_000 },
+	]);
+});
+
+describe("FileBrowser (slice 7a — TanStack DataTable parity)", () => {
+	it("renders the 5 locked columns (type/name/ext/size/modified)", () => {
+		render(<FileBrowser />);
+
+		const headers = screen
+			.getAllByRole("columnheader")
+			.map((h) => h.textContent);
+		// The leading selection column header is empty (checkbox); the 5 data
+		// columns are Type, Name, Ext, Size, Modified in that order.
+		expect(headers).toEqual(
+			expect.arrayContaining(["Type", "Name", "Ext", "Size", "Modified"]),
+		);
+		expect(headers.filter((h) => h === "Type").length).toBe(1);
+		expect(headers.filter((h) => h === "Modified").length).toBe(1);
+	});
+
+	it("clicking a file row selects it for ffprobe preview (Media info)", async () => {
+		render(<FileBrowser />);
+
+		// The selected-file path surfaces in the Browser status caption once chosen.
+		expect(screen.queryByText(/Selected: \/video\.mkv/)).toBeNull();
+
+		await userEvent.click(screen.getByText("video.mkv"));
+		expect(screen.getByText(/Selected: \/video\.mkv/)).toBeInTheDocument();
+
+		// A recognized video file enters the ffprobe branch; with empty ffprobe
+		// data it shows the "No ffprobe data available." status (proving the
+		// selected file routed into the Media info preview flow).
+		expect(screen.getByText("No ffprobe data available.")).toBeInTheDocument();
+	});
+
+	it("clicking a directory row navigates into it (no ffprobe selection)", async () => {
+		render(<FileBrowser />);
+
+		await userEvent.click(screen.getByText("movies"));
+		// After navigating into /movies, the status caption shows the new cwd and
+		// NO "Selected:" segment (directories are opened, not selected for preview).
+		expect(screen.getByText(/Current: \/movies\b/)).toBeInTheDocument();
+		expect(screen.queryByText(/Selected:/)).toBeNull();
+	});
+});