feat(meals): add optional recipe link to meal cards (#18)

- New optional recipe_url field in the meal modal (below Notes)
- Link icon appears on meal cards when a URL is set, opens in new tab
- DB migration v6: ALTER TABLE meals ADD COLUMN recipe_url TEXT
- API: recipe_url supported in POST /meals and PUT /meals/:id
- i18n: new keys recipeUrlLabel, recipeUrlPlaceholder, openRecipe (de, en, sv, it)

public/styles/meals.css

@@ -266,6 +266,14 @@
   color: var(--color-success);
 }
 
+.meal-card__action-btn--recipe {
+  text-decoration: none;
+}
+
+.meal-card__action-btn--recipe:hover {
+  color: var(--color-accent);
+}
+
 /* --------------------------------------------------------
  * Meals-Modal Content-Styles (Overlay/Panel via shared modal.js)
  * -------------------------------------------------------- */