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__/LibraryOverview.test.tsx

@@ -0,0 +1,35 @@
+import { describe, it, expect } from "vitest";
+import { render, screen } from "@testing-library/react";
+import { LibraryOverview } from "../LibraryOverview";
+import type { LibraryCount } from "../../types";
+
+const libraries: LibraryCount[] = [
+	{
+		library: "Films",
+		type: "movies",
+		movies: 100,
+		series: 0,
+		episodes: 0,
+		total: 100,
+	},
+	{
+		library: "Shows",
+		type: "tvshows",
+		movies: 0,
+		series: 12,
+		episodes: 240,
+		total: 252,
+	},
+];
+
+describe("LibraryOverview", () => {
+	it("renders movie and TV library cards with their counts", () => {
+		render(<LibraryOverview libraries={libraries} />);
+		expect(screen.getByText("Movie libraries")).toBeInTheDocument();
+		expect(screen.getByText("TV libraries")).toBeInTheDocument();
+		expect(screen.getByText("Films")).toBeInTheDocument();
+		expect(screen.getByText(/Total: 100 \| Movies: 100/)).toBeInTheDocument();
+		expect(screen.getByText("Shows")).toBeInTheDocument();
+		expect(screen.getByText(/Total: 252 \| Series: 12/)).toBeInTheDocument();
+	});
+});