refactor: convert favorites to Server Actions

- Add src/app/actions/favorites.ts with addToFavorites and removeFromFavorites
- Update SeriesHeader to use Server Actions instead of fetch
- Keep API route GET only (POST/DELETE removed)

src/app/actions/favorites.ts

@@ -0,0 +1,39 @@
+"use server";
+
+import { FavoriteService } from "@/lib/services/favorite.service";
+import { ERROR_CODES } from "@/constants/errorCodes";
+import { AppError } from "@/utils/errors";
+
+/**
+ * Ajoute une série aux favoris
+ */
+export async function addToFavorites(
+  seriesId: string
+): Promise<{ success: boolean; message: string }> {
+  try {
+    await FavoriteService.addToFavorites(seriesId);
+    return { success: true, message: "Série ajoutée aux favoris" };
+  } catch (error) {
+    if (error instanceof AppError) {
+      return { success: false, message: error.message };
+    }
+    return { success: false, message: "Erreur lors de l'ajout aux favoris" };
+  }
+}
+
+/**
+ * Retire une série des favoris
+ */
+export async function removeFromFavorites(
+  seriesId: string
+): Promise<{ success: boolean; message: string }> {
+  try {
+    await FavoriteService.removeFromFavorites(seriesId);
+    return { success: true, message: "Série retirée des favoris" };
+  } catch (error) {
+    if (error instanceof AppError) {
+      return { success: false, message: error.message };
+    }
+    return { success: false, message: "Erreur lors de la suppression des favoris" };
+  }
+}