feat: error banners and first-run navigation flow

tests/test_error_ux.py

@@ -0,0 +1,57 @@
+from __future__ import annotations
+
+import pytest
+from fastapi.testclient import TestClient
+
+from chat.app import app
+
+
+@pytest.fixture
+def client(tmp_path, monkeypatch):
+    cfg = tmp_path / "config.toml"
+    cfg.write_text('featherless_api_key = "test"\n')
+    monkeypatch.setenv("CHAT_CONFIG_PATH", str(cfg))
+    db = tmp_path / "test.db"
+    monkeypatch.setenv("CHAT_DB_PATH", str(db))
+    with TestClient(app) as c:
+        if hasattr(app.state, "background_worker"):
+            app.state.background_worker.enabled = False
+        yield c
+
+
+def _setup_minimal_state(db_path):
+    """Set up enough state so the first-run middleware doesn't redirect."""
+    from chat.db.connection import open_db
+    from chat.eventlog.log import append_event
+    from chat.eventlog.projector import project
+
+    with open_db(db_path) as conn:
+        append_event(
+            conn,
+            kind="you_authored",
+            payload={"name": "Me", "pronouns": "", "persona": ""},
+        )
+        append_event(
+            conn,
+            kind="bot_authored",
+            payload={
+                "id": "bot_a",
+                "name": "BotA",
+                "persona": "",
+                "voice_samples": [],
+                "traits": [],
+                "backstory": "",
+                "initial_relationship_to_you": "",
+                "kickoff_prose": "",
+            },
+        )
+        project(conn)
+
+
+def test_404_renders_friendly_page_for_html(client, tmp_path):
+    _setup_minimal_state(tmp_path / "test.db")
+    response = client.get("/chats/no_such_chat")
+    assert response.status_code == 404
+    body = response.text
+    assert "404" in body
+    assert "back to" in body.lower()