feat: Implement Cronitor health monitor

src/contexts/CronitorHealthContext.tsx

@@ -0,0 +1,63 @@
+import { createContext, useContext, ReactNode, useState, useEffect } from 'react';
+import { useCronitorHealth as useCronitorHealthQuery } from '@/hooks/useCronitorHealth';
+
+interface CronitorHealthContextType {
+  passing: boolean | null; // null = loading/unknown
+  isLoading: boolean;
+  error: Error | null;
+  lastChecked: Date | null;
+  isBannerDismissed: boolean;
+  dismissBanner: () => void;
+}
+
+const CronitorHealthContext = createContext<CronitorHealthContextType | undefined>(undefined);
+
+const DISMISSAL_DURATION = 15 * 60 * 1000; // 15 minutes
+const DISMISSAL_KEY = 'cronitor-banner-dismissed';
+
+export function CronitorHealthProvider({ children }: { children: ReactNode }) {
+  const { data, isLoading, error } = useCronitorHealthQuery();
+  const [dismissedUntil, setDismissedUntil] = useState<number | null>(() => {
+    const stored = localStorage.getItem(DISMISSAL_KEY);
+    return stored ? parseInt(stored) : null;
+  });
+
+  const dismissBanner = () => {
+    const until = Date.now() + DISMISSAL_DURATION;
+    localStorage.setItem(DISMISSAL_KEY, until.toString());
+    setDismissedUntil(until);
+  };
+
+  const isBannerDismissed = dismissedUntil ? Date.now() < dismissedUntil : false;
+  
+  // Auto-clear dismissal when API is healthy again
+  useEffect(() => {
+    if (data?.passing === true && dismissedUntil) {
+      localStorage.removeItem(DISMISSAL_KEY);
+      setDismissedUntil(null);
+    }
+  }, [data?.passing, dismissedUntil]);
+
+  return (
+    <CronitorHealthContext.Provider
+      value={{
+        passing: data?.passing ?? null,
+        isLoading,
+        error: error as Error | null,
+        lastChecked: data ? new Date() : null,
+        isBannerDismissed,
+        dismissBanner,
+      }}
+    >
+      {children}
+    </CronitorHealthContext.Provider>
+  );
+}
+
+export function useCronitorHealth() {
+  const context = useContext(CronitorHealthContext);
+  if (context === undefined) {
+    throw new Error('useCronitorHealth must be used within CronitorHealthProvider');
+  }
+  return context;
+}