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/locales/en.json

@@ -234,7 +234,10 @@
     "ingredientCount": "{{count}} ingredient",
     "ingredientCountPlural": "{{count}} ingredients",
     "titleRequired": "Title is required",
-    "loadingIndicator": "Loading…"
+    "loadingIndicator": "Loading…",
+    "recipeUrlLabel": "Recipe link (optional)",
+    "recipeUrlPlaceholder": "https://…",
+    "openRecipe": "Open recipe"
   },
 
   "calendar": {