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/pages/BookingCancelled.tsx

@@ -1,3 +1,10 @@
+const STRINGS = {
+  heading: "Appointment Cancelled",
+  body: "Your appointment has been cancelled. If this was a mistake or you'd like to rebook, please contact us.",
+  bookAgain: "Book again",
+  backToPortal: "Back to Portal",
+} as const;
+
 export function BookingCancelledPage() {
   return (
     <div
@@ -7,12 +14,12 @@ export function BookingCancelledPage() {
         alignItems: "center",
         justifyContent: "center",
         fontFamily: "system-ui, sans-serif",
-        background: "#fff7ed",
+        background: "var(--color-cancelled-bg)",
       }}
     >
       <div
         style={{
-          background: "#fff",
+          background: "var(--color-surface)",
           borderRadius: 12,
           padding: "2.5rem 3rem",
           boxShadow: "0 4px 24px rgba(0,0,0,0.08)",
@@ -21,28 +28,45 @@ export function BookingCancelledPage() {
         }}
       >
         <div style={{ fontSize: 56, marginBottom: "0.5rem" }}>✗</div>
-        <h1 style={{ color: "#c2410c", fontSize: 24, margin: "0 0 0.5rem" }}>
-          Appointment Cancelled
+        <h1 style={{ color: "var(--color-cancelled-dark)", fontSize: 24, margin: "0 0 0.5rem" }}>
+          {STRINGS.heading}
         </h1>
-        <p style={{ color: "#4b5563", margin: "0 0 1.5rem" }}>
-          Your appointment has been cancelled. If this was a mistake or you'd
-          like to rebook, please contact us.
+        <p style={{ color: "var(--color-text-secondary)", margin: "0 0 1.5rem" }}>
+          {STRINGS.body}
         </p>
-        <a
-          href="/"
-          style={{
-            display: "inline-block",
-            padding: "0.6rem 1.5rem",
-            background: "#ea580c",
-            color: "#fff",
-            borderRadius: 6,
-            textDecoration: "none",
-            fontWeight: 600,
-            fontSize: 14,
-          }}
-        >
-          Back to Portal
-        </a>
+
+        <div style={{ display: "flex", flexDirection: "column", gap: "0.75rem", alignItems: "center" }}>
+          <a
+            href="/admin/book"
+            style={{
+              display: "inline-block",
+              padding: "0.6rem 1.5rem",
+              background: "var(--color-primary)",
+              color: "#fff",
+              borderRadius: 6,
+              textDecoration: "none",
+              fontWeight: 600,
+              fontSize: 14,
+            }}
+          >
+            {STRINGS.bookAgain}
+          </a>
+          <a
+            href="/"
+            style={{
+              display: "inline-block",
+              padding: "0.6rem 1.5rem",
+              background: "var(--color-cancelled)",
+              color: "#fff",
+              borderRadius: 6,
+              textDecoration: "none",
+              fontWeight: 600,
+              fontSize: 14,
+            }}
+          >
+            {STRINGS.backToPortal}
+          </a>
+        </div>
       </div>
     </div>
   );