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/app/(app)/recipes/page.tsx

@@ -7,6 +7,7 @@ import { eq, desc, asc, and, ilike, or, inArray } from "@epicure/db";
 import { RecipesHeader } from "@/components/recipe/recipes-header";
 import { RecipesEmptyState } from "@/components/recipe/recipes-empty-state";
 import { RecipesGrid } from "@/components/recipe/recipes-grid";
+import { CookingAssistantPanel } from "@/components/recipe/cooking-assistant-panel";
 import { getMessages } from "@/lib/i18n/server";
 
 export const metadata: Metadata = {};
@@ -139,6 +140,8 @@ export default async function RecipesPage({ searchParams }: { searchParams: Sear
           )}
         </div>
       )}
+
+      <CookingAssistantPanel />
     </div>
   );
 }