refactor(monitoring): remove orphaned frontend DiskSpaceCard (slice 2)

After slice 1 removed /api/monitoring/disk, the frontend DiskSpaceCard
component (and its DiskSpace type) had zero importers — it fed nothing.
Delete them.

Removed (frontend):
- components/DiskSpaceCard.tsx
- components/__tests__/DiskSpaceCard.test.tsx
- types/index.ts: DiskSpace interface

Gate: build + lint + vitest (22 files / 63 tests) green.

frontend/src/components/__tests__/DiskSpaceCard.test.tsx

@@ -1,22 +0,0 @@
-import { describe, it, expect } from "vitest";
-import { render, screen } from "@testing-library/react";
-import { DiskSpaceCard } from "../DiskSpaceCard";
-
-describe("DiskSpaceCard", () => {
-	it("preserves the used / free / total breakdown and percent headline", () => {
-		render(
-			<DiskSpaceCard
-				used={500000000000}
-				available={500000000000}
-				size={1000000000000}
-				usedPct="50"
-			/>,
-		);
-		expect(screen.getByText(/50 used/i)).toBeInTheDocument();
-		expect(screen.getByText("Used")).toBeInTheDocument();
-		expect(screen.getByText("Free")).toBeInTheDocument();
-		expect(screen.getByText("Total")).toBeInTheDocument();
-		// Disk space label
-		expect(screen.getByText(/disk space/i)).toBeInTheDocument();
-	});
-});