feat: implement Favorites feature with persistent storage and sidebar integration and update lake data.

src/hooks/useFavorites.tsx

@@ -0,0 +1,46 @@
+import { createContext, useContext, useState, useCallback, type ReactNode } from 'react';
+
+const STORAGE_KEY = 'hladinator_favorites';
+
+const loadFavorites = (): string[] => {
+  try {
+    const raw = localStorage.getItem(STORAGE_KEY);
+    return raw ? JSON.parse(raw) : [];
+  } catch {
+    return [];
+  }
+};
+
+interface FavoritesContextType {
+  favorites: string[];
+  toggleFavorite: (id: string) => void;
+  isFavorite: (id: string) => boolean;
+}
+
+const FavoritesContext = createContext<FavoritesContextType>({
+  favorites: [],
+  toggleFavorite: () => {},
+  isFavorite: () => false,
+});
+
+export const FavoritesProvider = ({ children }: { children: ReactNode }) => {
+  const [favorites, setFavorites] = useState<string[]>(loadFavorites);
+
+  const toggleFavorite = useCallback((id: string) => {
+    setFavorites(prev => {
+      const next = prev.includes(id) ? prev.filter(f => f !== id) : [...prev, id];
+      localStorage.setItem(STORAGE_KEY, JSON.stringify(next));
+      return next;
+    });
+  }, []);
+
+  const isFavorite = useCallback((id: string) => favorites.includes(id), [favorites]);
+
+  return (
+    <FavoritesContext.Provider value={{ favorites, toggleFavorite, isFavorite }}>
+      {children}
+    </FavoritesContext.Provider>
+  );
+};
+
+export const useFavorites = () => useContext(FavoritesContext);