Add the assistant: a chat that drives the MCP tools in-process

A provider set under Settings (OpenAI-compatible; key encrypted in
flow-data/ai.json) turns on an Assistant panel in the sidebar. Each turn runs
the FastMCP tools in-process as the person chatting, streams the rounds over
SSE and keeps one transcript per user. The panel floats on any screen and
shares the editor's lane: a settings panel keeps the top half, the chat the
bottom. The MCP client is now built once, for the endpoint and the chat.

docs/interface/index.md

@@ -25,7 +25,7 @@ phone the sidebar collapses to a sheet.
 | **Alerts** | where failures get sent |
 | **Admin** | users (superusers only) |
 | **Search** | anything in this instance, by name |
-| **Settings** | your account, appearance, and remote access |
+| **Settings** | your account, appearance, remote access, and the assistant |
 
 ### Search
 

docs/interface/operations.md

@@ -129,3 +129,10 @@ follow the system).
 **Remote access** is where an instance is paired with a portal, remote
 users are admitted, and the link is cut again. That has [its own
 page](portal.md).
+
+**Assistant** turns on the chat that builds and edits flows: pick a provider,
+paste a key, fetch the model list and choose one. The key is stored encrypted
+on the instance and never shown again. One provider serves everyone who signs
+in here, and until one is set nothing in the interface mentions the feature.
+How the chat works, and what it may do, is on the [agents
+page](../code/agents.md#the-assistant).