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__/TabbedCard.test.tsx

@@ -0,0 +1,32 @@
+import { describe, it, expect, vi } from "vitest";
+import { render, screen } from "@testing-library/react";
+import userEvent from "@testing-library/user-event";
+import { TabbedCard } from "../TabbedCard";
+import { TabsTrigger } from "@/components/ui/tabs";
+
+describe("TabbedCard", () => {
+	it("renders the provided tab triggers and reports selection changes", async () => {
+		const onChange = vi.fn();
+		render(
+			<TabbedCard
+				value="jellyfin"
+				onChange={onChange}
+				tabs={[
+					<TabsTrigger key="jellyfin" value="jellyfin">
+						Jellyfin
+					</TabsTrigger>,
+					<TabsTrigger key="nextcloud" value="nextcloud">
+						Nextcloud
+					</TabsTrigger>,
+				]}
+			>
+				<p>Body</p>
+			</TabbedCard>,
+		);
+		expect(screen.getByText("Jellyfin")).toBeInTheDocument();
+		expect(screen.getByText("Body")).toBeInTheDocument();
+
+		await userEvent.click(screen.getByText("Nextcloud"));
+		expect(onChange).toHaveBeenCalledWith("nextcloud");
+	});
+});