feat: general cooking-question chatbot on the recipes homepage

The existing recipe chat (RecipeChatPanel) is tightly scoped to one
recipe — recipeId is required and its system prompt embeds that recipe's
full ingredient/step context. Added a sibling, not a modification: a new
floating assistant on the recipes list page for questions that aren't
about any specific recipe (technique, substitutions, timing, food safety).

New /api/v1/ai/cooking-chat route (same quota/rate-limit/model-resolution
pattern as recipe-chat, minus the recipe lookup) and CookingAssistantPanel
component (same floating-button + Sheet UI). Positioned at bottom-24
rather than bottom-6 so it doesn't sit in the same band as the recipe
list's floating bulk-action bar during select mode.

Verified locally: asked a real cooking question through the actual UI,
got a correctly-formatted markdown answer with no recipe context leaking
in (confirmed the request payload only carries the question, no recipeId).

apps/web/messages/fr.json

@@ -388,6 +388,20 @@
     "inputPlaceholder": "Posez une question…",
     "send": "Envoyer le message"
   },
+  "cookingChat": {
+    "sorry": "Désolé, je n'ai pas pu répondre à cela.",
+    "error": "Une erreur s'est produite. Veuillez réessayer.",
+    "suggestion1": "Comment savoir si l'huile est assez chaude ?",
+    "suggestion2": "Quel est un bon substitut au lait fermenté ?",
+    "suggestion3": "Combien de temps puis-je garder des restes au frigo ?",
+    "suggestion4": "Quelle est la différence entre mijoter et bouillir ?",
+    "ariaLabel": "Poser une question de cuisine",
+    "title": "Assistant culinaire",
+    "subtitle": "Posez n'importe quelle question de cuisine — pas liée à une recette précise",
+    "intro": "Demandez n'importe quoi sur la cuisine — techniques, substitutions, minutage, équipement…",
+    "inputPlaceholder": "Posez une question…",
+    "send": "Envoyer le message"
+  },
   "servingScaler": {
     "servings": "Portions",
     "reset": "Réinitialiser",