Whole chat history with infinite scroll back; fix switching between sidebar tabs

Chat is an append-only chat.jsonl paged by sequence number: the welcome
carries the latest 50 messages and GET /rigshare/api/chat pages further back.
The panel loads older pages as you scroll up, keeps your place, and adds date
dividers. ComfyUI reuses one slot element between custom sidebar tabs, so
each tab now replaces the slot's content instead of appending to it.

tests/python/test_server.py

@@ -292,3 +292,35 @@ def test_moves_follow_open_tabs(run):
             for sock in socks.values():
                 await sock.close()
     run(scenario())
+
+
+def test_chat_history_pages(run):
+    async def scenario():
+        async with rig_server() as rig:
+            await rig.setup_admin()
+            sock = await rig.ws("boss")
+            for i in range(70):
+                await sock.send({"type": "chat", "text": f"hello {i}"})
+            await asyncio.sleep(0.3)
+            await sock.close()
+            sock = await rig.ws("boss")
+            await sock.close()
+            # The welcome carries only the newest page; the rest comes over HTTP.
+            h = rig.headers["boss"]
+            session = rig.http
+            ws = await session.ws_connect(rig.url("/rigshare/ws"), headers=h)
+            await ws.send_json({"type": "hello"})
+            while True:
+                welcome = json.loads((await ws.receive()).data)
+                if welcome["type"] == "welcome":
+                    break
+            await ws.close()
+            assert len(welcome["chat"]) == 50 and welcome["chat_more"]
+            oldest = welcome["chat"][0]["seq"]
+            older = await (await session.get(rig.url(f"/rigshare/api/chat?before={oldest}&limit=50"), headers=h)).json()
+            texts = [m["text"] for m in older["messages"]]
+            assert not older["more"] and older["messages"][0]["seq"] == 0
+            seqs = [m["seq"] for m in older["messages"] + welcome["chat"]]
+            assert "hello 0" in texts and seqs == list(range(len(seqs))), "no gaps, no repeats"
+            assert (await session.get(rig.url("/rigshare/api/chat?before=x"))).status in (400, 401)
+    run(scenario())