feat(GRO-1792): add recovery paths to booking error and cancellation pages

- Add "Start a new booking" button to BookingError linking to /admin/book
- Add "Book again" button to BookingCancelled linking to /admin/book
- Add business contact info section to BookingError (from BUSINESS_CONTACT_INFO constant)
- Replace hardcoded colors with CSS variables (--color-error, --color-cancelled, etc.)
- Add page-level string constants to eliminate hardcoded strings
- Add unit tests for both pages (9 tests passing)

Co-Authored-By: Paperclip <noreply@paperclip.ing>

src/__tests__/BookingCancelled.test.tsx

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+import { describe, it, expect } from "vitest";
+import { render, screen } from "@testing-library/react";
+import { BookingCancelledPage } from "../pages/BookingCancelled.tsx";
+
+describe("BookingCancelledPage", () => {
+  it("renders the cancelled heading", () => {
+    render(<BookingCancelledPage />);
+    expect(screen.getByRole("heading", { name: /Appointment Cancelled/i })).toBeInTheDocument();
+  });
+
+  it("renders the cancelled body text", () => {
+    render(<BookingCancelledPage />);
+    expect(screen.getByText(/Your appointment has been cancelled/i)).toBeInTheDocument();
+  });
+
+  it("has a Book again link pointing to /admin/book", () => {
+    render(<BookingCancelledPage />);
+    const link = screen.getByRole("link", { name: /Book again/i });
+    expect(link).toHaveAttribute("href", "/admin/book");
+  });
+
+  it("has a Back to Portal link pointing to /", () => {
+    render(<BookingCancelledPage />);
+    const link = screen.getByRole("link", { name: /Back to Portal/i });
+    expect(link).toHaveAttribute("href", "/");
+  });
+});

src/__tests__/BookingError.test.tsx

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+import { describe, it, expect } from "vitest";
+import { render, screen } from "@testing-library/react";
+import { BookingErrorPage } from "../pages/BookingError.tsx";
+import { BUSINESS_CONTACT_INFO } from "../lib/contact.ts";
+
+describe("BookingErrorPage", () => {
+  it("renders the error heading", () => {
+    render(<BookingErrorPage />);
+    expect(screen.getByRole("heading", { name: /Link Invalid or Expired/i })).toBeInTheDocument();
+  });
+
+  it("renders the error body text", () => {
+    render(<BookingErrorPage />);
+    expect(screen.getByText(/This confirmation link is invalid/i)).toBeInTheDocument();
+  });
+
+  it("has a Start a new booking link pointing to /admin/book", () => {
+    render(<BookingErrorPage />);
+    const link = screen.getByRole("link", { name: /Start a new booking/i });
+    expect(link).toHaveAttribute("href", "/admin/book");
+  });
+
+  it("has a Back to Portal link pointing to /", () => {
+    render(<BookingErrorPage />);
+    const link = screen.getByRole("link", { name: /Back to Portal/i });
+    expect(link).toHaveAttribute("href", "/");
+  });
+
+  it("displays business contact phone", () => {
+    render(<BookingErrorPage />);
+    expect(screen.getByText(BUSINESS_CONTACT_INFO.phone)).toBeInTheDocument();
+  });
+
+  it("displays business contact email", () => {
+    render(<BookingErrorPage />);
+    expect(screen.getByText(BUSINESS_CONTACT_INFO.email)).toBeInTheDocument();
+  });
+});