feat: move tailscale device discovery and settings into a plugin

Adds plugins/tailscale/ with a devices sidebar tab, admin settings
section, and re-auth dialog. Host Metrics tailscale manager card and
SSH re-auth banner parsing stay in core.

src/ui/tests/plugins/tailscale/AdminTailscaleSection.test.tsx

@@ -0,0 +1,62 @@
+import { describe, expect, it, vi } from "vitest";
+import { render, screen } from "@testing-library/react";
+import userEvent from "@testing-library/user-event";
+import { AdminTailscaleSection } from "../../../../../plugins/tailscale/frontend/AdminTailscaleSection";
+
+vi.mock("react-i18next", () => ({
+  useTranslation: () => ({ t: (key: string) => key }),
+}));
+
+describe("AdminTailscaleSection", () => {
+  it("renders the API key and base URL fields and saves on click", async () => {
+    const user = userEvent.setup();
+    const handleSave = vi.fn();
+    render(
+      <AdminTailscaleSection
+        open
+        onToggle={vi.fn()}
+        tailscaleApiKey="tskey-api-abc"
+        setTailscaleApiKey={vi.fn()}
+        tailscaleApiBaseUrl="https://api.tailscale.com/api/v2"
+        setTailscaleApiBaseUrl={vi.fn()}
+        handleSaveTailscaleApiKey={handleSave}
+      />,
+    );
+
+    expect(
+      (
+        screen.getByPlaceholderText(
+          "tskey-api-... / hskey-api-...",
+        ) as HTMLInputElement
+      ).value,
+    ).toBe("tskey-api-abc");
+    expect(
+      (
+        screen.getByPlaceholderText(
+          "https://api.tailscale.com/api/v2",
+        ) as HTMLInputElement
+      ).value,
+    ).toBe("https://api.tailscale.com/api/v2");
+
+    await user.click(screen.getByText("common.save"));
+    expect(handleSave).toHaveBeenCalledOnce();
+  });
+
+  it("does not render its content when closed", () => {
+    render(
+      <AdminTailscaleSection
+        open={false}
+        onToggle={vi.fn()}
+        tailscaleApiKey=""
+        setTailscaleApiKey={vi.fn()}
+        tailscaleApiBaseUrl=""
+        setTailscaleApiBaseUrl={vi.fn()}
+        handleSaveTailscaleApiKey={vi.fn()}
+      />,
+    );
+
+    expect(
+      screen.queryByPlaceholderText("tskey-api-... / hskey-api-..."),
+    ).toBeNull();
+  });
+});