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__/SectionCard.test.tsx

@@ -0,0 +1,27 @@
+import { describe, it, expect } from "vitest";
+import { render, screen } from "@testing-library/react";
+import { SectionCard } from "../SectionCard";
+
+describe("SectionCard", () => {
+	it("renders title, description, action, and children", () => {
+		render(
+			<SectionCard
+				title="Shortcuts"
+				description="Quick links"
+				action={<button type="button">Add</button>}
+			>
+				<p>Body content</p>
+			</SectionCard>,
+		);
+		expect(screen.getByText("Shortcuts")).toBeInTheDocument();
+		expect(screen.getByText("Quick links")).toBeInTheDocument();
+		expect(screen.getByRole("button", { name: "Add" })).toBeInTheDocument();
+		expect(screen.getByText("Body content")).toBeInTheDocument();
+	});
+
+	it("renders without a description or action", () => {
+		render(<SectionCard title="Only title">children</SectionCard>);
+		expect(screen.getByText("Only title")).toBeInTheDocument();
+		expect(screen.getByText("children")).toBeInTheDocument();
+	});
+});