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/en.json

@@ -388,6 +388,20 @@
     "inputPlaceholder": "Ask a question…",
     "send": "Send message"
   },
+  "cookingChat": {
+    "sorry": "Sorry, I couldn't answer that.",
+    "error": "Something went wrong. Please try again.",
+    "suggestion1": "How do I know when oil is hot enough?",
+    "suggestion2": "What's a good substitute for buttermilk?",
+    "suggestion3": "How long can I keep leftovers in the fridge?",
+    "suggestion4": "What's the difference between simmering and boiling?",
+    "ariaLabel": "Ask a cooking question",
+    "title": "Cooking assistant",
+    "subtitle": "Ask any cooking question — not tied to a specific recipe",
+    "intro": "Ask anything about cooking — techniques, substitutions, timing, equipment…",
+    "inputPlaceholder": "Ask a question…",
+    "send": "Send message"
+  },
   "servingScaler": {
     "servings": "Servings",
     "reset": "Reset",