feat: ChatGPT/Claude-style persistent sidebar for desktop fullscreen chat (v0.57.0)

Cooking assistant's fullscreen mode gets a real left sidebar (always-
visible conversation list, hover-reveal rename/delete) on desktop
(md+), replacing the small dropdown menu that made sense in the narrow
420px drawer but not in a full-viewport layout. Mobile fullscreen and
the normal drawer keep the dropdown (ConversationMenu) — no room for a
persistent sidebar there.

Extracted the shared fetch/rename/delete/create logic into
use-conversation-list.ts so the dropdown (ConversationMenu) and the new
sidebar (ConversationSidebar) can't silently diverge — both are thin
render layers over the same hook. ConversationMenu gained an optional
className prop so the panel can hide it with `md:hidden` specifically
when the sidebar is already covering that job.

Per-recipe chat is untouched — it's single-threaded by design and never
used either of these components.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "epicure",
-  "version": "0.56.0",
+  "version": "0.57.0",
   "private": true,
   "scripts": {
     "dev": "pnpm --filter web dev",