feat(frontend): slice 2 — migrate 11 shared components to shadcn/Tailwind

Web UI rework. Shared-components slice (drift prevention):
- Migrate SectionCard, SelectionRailCard, TabbedCard, MetricCard,
  DiskSpaceCard, HoverEditButton, DialogFooter, ConfirmDialog,
  LibraryOverview, NowPlaying, SessionActivityPanel off @mui
- HoverEditButton: MUI IconButton + EditOutlined -> Button + lucide Pencil
- Status mapping uses the success Badge variant (chart-2) for healthy
- Exported APIs preserved so consuming pages still compile (no page edits)
- 11 behavioral Vitest component tests added

Gate: build + lint + test green.

frontend/src/components/__tests__/ConfirmDialog.test.tsx

@@ -0,0 +1,40 @@
+import { describe, it, expect, vi } from "vitest";
+import { render, screen } from "@testing-library/react";
+import userEvent from "@testing-library/user-event";
+import { ConfirmDialog } from "../ConfirmDialog";
+
+describe("ConfirmDialog", () => {
+	it("renders the title and message and wires confirm/cancel", async () => {
+		const onCancel = vi.fn();
+		const onConfirm = vi.fn();
+		render(
+			<ConfirmDialog
+				open
+				title="Delete machine?"
+				message="This cannot be undone."
+				confirmLabel="Delete"
+				onCancel={onCancel}
+				onConfirm={onConfirm}
+			/>,
+		);
+		expect(screen.getByText("Delete machine?")).toBeInTheDocument();
+		expect(screen.getByText("This cannot be undone.")).toBeInTheDocument();
+		await userEvent.click(screen.getByRole("button", { name: "Delete" }));
+		expect(onConfirm).toHaveBeenCalledTimes(1);
+		await userEvent.click(screen.getByRole("button", { name: "Cancel" }));
+		expect(onCancel).toHaveBeenCalledTimes(1);
+	});
+
+	it("renders nothing when closed", () => {
+		render(
+			<ConfirmDialog
+				open={false}
+				title="Hidden"
+				message="nope"
+				onCancel={() => {}}
+				onConfirm={() => {}}
+			/>,
+		);
+		expect(screen.queryByText("Hidden")).not.toBeInTheDocument();
+	});
+});