feat(appkit): use chat history in the full chat UI

[hubertzub-db]

Why

<ChatApp> was a single-conversation shell — no way to revisit a past thread or start a fresh one. The thread-history hooks added in the previous PR were only wired into the build-your-own demo.

What changed

• <ChatApp> now ships a collapsible history sidebar (history prop, default true; pass false for the old single-conversation layout).
• Sidebar features: new-chat button, icon-mode collapse, per-thread delete with confirm dialog, date-grouped list (Today / Yesterday / Last 7 / Last 30 / Older).
• Switching threads re-keys the inner ChatBody so useChat reinitializes with seeded messages — mirrors the manual pattern already used in agent.route.tsx.
• Sidebar uses inline flow (no position: fixed), so <ChatApp> fits inside a parent below a host header without overlapping it. The dev-playground showcase route was retightened accordingly (h-[calc(100svh-65px)]).
• Radix popovers (tooltip/dropdown/alert-dialog) are portaled into the chat boundary so they inherit the chat-scoped theme. Open/close animations are disabled for popovers inside that boundary to match the rest of the chat surface.

full-history.mov

Tests

• Unit tests
• Tested manually (see above)