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__/DiskSpaceCard.test.tsx

@@ -0,0 +1,22 @@
+import { describe, it, expect } from "vitest";
+import { render, screen } from "@testing-library/react";
+import { DiskSpaceCard } from "../DiskSpaceCard";
+
+describe("DiskSpaceCard", () => {
+	it("preserves the used / free / total breakdown and percent headline", () => {
+		render(
+			<DiskSpaceCard
+				used={500000000000}
+				available={500000000000}
+				size={1000000000000}
+				usedPct="50"
+			/>,
+		);
+		expect(screen.getByText(/50 used/i)).toBeInTheDocument();
+		expect(screen.getByText("Used")).toBeInTheDocument();
+		expect(screen.getByText("Free")).toBeInTheDocument();
+		expect(screen.getByText("Total")).toBeInTheDocument();
+		// Disk space label
+		expect(screen.getByText(/disk space/i)).toBeInTheDocument();
+	});
+});