feat: multiple named conversations for the cooking assistant

The general assistant had exactly one conversation per user forever
(recipeId null on chat_messages) — no way to start fresh or organize
by topic. Adds an ai_conversations table (title, timestamps) and a
nullable conversationId FK on chat_messages; the assistant panel gets
a conversations menu to create/switch/rename/delete, auto-titling a
new conversation from its first question.

Per-recipe chat is untouched — each recipe already has one natural
thread, so multi-conversation support only applies to the homepage
assistant. Pre-existing general messages (no conversationId) aren't
migrated into the new model and won't appear in the conversation list.

Requires migration 0042 to run against a live DB — not applied in
this sandbox (no Docker here); run `pnpm db:migrate`.

v0.43.0

apps/web/messages/en.json

@@ -461,6 +461,24 @@
     "inputPlaceholder": "Ask a question…",
     "send": "Send message"
   },
+  "conversations": {
+    "menuAriaLabel": "Conversations",
+    "newConversation": "New conversation",
+    "loading": "Loading…",
+    "empty": "No conversations yet.",
+    "untitled": "Untitled",
+    "renameSave": "Save",
+    "renameCancel": "Cancel",
+    "renameAriaLabel": "Rename",
+    "deleteAriaLabel": "Delete",
+    "deleteConfirmTitle": "Delete this conversation?",
+    "deleteConfirmDescription": "This deletes the conversation and all its messages. This can't be undone.",
+    "deleteCancel": "Cancel",
+    "deleteConfirm": "Delete",
+    "createFailed": "Failed to start a new conversation",
+    "renameFailed": "Failed to rename conversation",
+    "deleteFailed": "Failed to delete conversation"
+  },
   "servingScaler": {
     "servings": "Servings",
     "reset": "Reset",