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__/NowPlaying.test.tsx

@@ -0,0 +1,12 @@
+import { describe, it, expect } from "vitest";
+import { render, screen } from "@testing-library/react";
+import { NowPlaying } from "../NowPlaying";
+
+describe("NowPlaying", () => {
+	it("renders the dashboard empty-state message contract when there are no sessions", () => {
+		render(<NowPlaying sessions={[]} />);
+		expect(
+			screen.getByText("No recent user activity sessions right now."),
+		).toBeInTheDocument();
+	});
+});