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__/MetricCard.test.tsx

@@ -0,0 +1,21 @@
+import { describe, it, expect } from "vitest";
+import { render, screen } from "@testing-library/react";
+import { MetricCard } from "../MetricCard";
+
+describe("MetricCard", () => {
+	it("renders the label, value, and subtext on the comfortable ramp", () => {
+		render(
+			<MetricCard label="Movies" value="1,234" subtext="across 3 libraries" />,
+		);
+		expect(screen.getByText("Movies")).toBeInTheDocument();
+		expect(screen.getByText("1,234")).toBeInTheDocument();
+		expect(screen.getByText(/across 3 libraries/)).toBeInTheDocument();
+	});
+
+	it("omits subtext when not provided", () => {
+		render(<MetricCard label="Series" value="42" />);
+		expect(screen.getByText("Series")).toBeInTheDocument();
+		expect(screen.getByText("42")).toBeInTheDocument();
+		expect(screen.queryByText(/subtext/i)).not.toBeInTheDocument();
+	});
+});