phase 19.0: built-in browser chat client at /chat

closes the "anyone can use without installing other tools" gap. hub
now serves a self-contained chat UI (~16 KB, zero deps) at /chat that
anyone can open in a browser to chat against any zhub-published AI.

new: zhub/chat.html
  - same dark + cyan + glassmorphism palette as the operator dashboard
  - auto-detect: when this hub has exactly one publisher, prefills the
    base URL with <origin>/<name>/v1 by hitting /api/dashboard
  - otherwise: form prompts for Base URL + API Key, persisted to
    localStorage so users don't paste each visit
  - streaming SSE chat with a clean message stream (user / ai / tool /
    error roles, color-coded with neon glows)
  - tool_call deltas surface as their own message bubbles in real time
    (works with stream=true tool-call streaming from Phase 4.2b)
  - composer area sticky at the bottom; Enter to send, Shift+Enter for
    newline; auto-resizing textarea
  - connection state indicator (ready/streaming/disconnected)

dashboard navbar gets a `chat` link so operators see it.

new: GET /chat endpoint serves the file (parallel to /).

178/178 pytest in isolation (same handful of cross-test flakes in
long-suite runs as before, all related to per-publisher state
isolation in module-scoped fixtures, not to this change). CI runs
cleanly on faster runners.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Zawwarsami16

README.md

@@ -143,6 +143,7 @@ Same URL + `zk_` key works in:
 
 | Surface | How |
 |---|---|
+| **Built-in chat at `<hub>/chat`** | No install needed. Open in any browser, paste URL+key (auto-detected on single-publisher hubs), chat. SSE streaming, localStorage-persisted config. |
 | [**Pocket**](https://github.com/Zawwarsami16/pocket) | First-class zhub provider — auto-detects `zk_` keys |
 | **TypingMind / OpenWebUI / LibreChat / Chatbox / Msty / Jan.ai / BoltAI / AnythingLLM** | "Custom OpenAI provider" — paste base URL + key |
 | **Cursor / Continue.dev** | Custom OpenAI base URL setting |

tests/test_chat_ui.py

@@ -0,0 +1,61 @@
+"""Phase 19.0 — built-in chat UI served at /chat."""
+
+import asyncio
+import socket
+import threading
+import time
+
+import pytest
+
+try:
+    import fastapi  # noqa
+    import uvicorn  # noqa
+    import httpx  # noqa
+    DEPS_AVAILABLE = True
+except ImportError:
+    DEPS_AVAILABLE = False
+
+if DEPS_AVAILABLE:
+    from zhub.server import create_app
+
+
+def _free_port() -> int:
+    with socket.socket() as s:
+        s.bind(("", 0))
+        return s.getsockname()[1]
+
+
+@pytest.fixture
+def hub():
+    if not DEPS_AVAILABLE:
+        pytest.skip("fastapi/uvicorn/httpx not installed")
+    port = _free_port()
+    app = create_app()
+
+    def run():
+        config = uvicorn.Config(app, host="127.0.0.1", port=port,
+                                log_level="warning")
+        asyncio.run(uvicorn.Server(config).serve())
+
+    threading.Thread(target=run, daemon=True).start()
+    for _ in range(30):
+        try:
+            with socket.create_connection(("127.0.0.1", port), timeout=0.1):
+                break
+        except OSError:
+            time.sleep(0.1)
+    yield port
+
+
+@pytest.mark.asyncio
+async def test_chat_ui_served_at_chat(hub):
+    async with httpx.AsyncClient(timeout=5.0) as c:
+        r = await c.get(f"http://127.0.0.1:{hub}/chat")
+    assert r.status_code == 200
+    assert "text/html" in r.headers["content-type"].lower()
+    body = r.text
+    # Key UI structures must be present
+    assert "zhub" in body.lower()
+    for needle in ["base url", "api key", "/chat/completions", "input",
+                   "messages"]:
+        assert needle in body.lower(), f"missing {needle!r}"