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__/HoverEditButton.test.tsx

@@ -0,0 +1,21 @@
+import { describe, it, expect, vi } from "vitest";
+import { render, screen } from "@testing-library/react";
+import userEvent from "@testing-library/user-event";
+import { HoverEditButton } from "../HoverEditButton";
+
+describe("HoverEditButton", () => {
+	it("fires onClick and exposes the default aria-label", async () => {
+		const onClick = vi.fn();
+		render(<HoverEditButton onClick={onClick} />);
+		const button = screen.getByRole("button", { name: "Edit" });
+		await userEvent.click(button);
+		expect(onClick).toHaveBeenCalledTimes(1);
+	});
+
+	it("honors a custom label", () => {
+		render(<HoverEditButton onClick={() => {}} label="Rename machine" />);
+		expect(
+			screen.getByRole("button", { name: "Rename machine" }),
+		).toBeInTheDocument();
+	});
+});