Add per-chat settings UI in web app for additional system prompt and tool checkboxes

docs/api/streaming-chat.md

@@ -49,6 +49,8 @@ Authentication:
       ]
     }
   ],
+  "additionalSystemPrompt": "optional one-off system prompt",
+  "enabledTools": ["web_search", "fetch_url"],
   "temperature": 0.2,
   "maxTokens": 256
 }
@@ -60,6 +62,8 @@ Notes:
 - If `chatId` is provided, backend validates it exists.
 - If `persist` is `false`, `chatId` must be omitted. Backend does not create a chat and does not persist input messages, tool-call messages, assistant output, or `LlmCall` metadata.
 - For persisted streams, backend stores only new non-assistant input history rows to avoid duplicates.
+- `additionalSystemPrompt`, when present directly or loaded from stored chat settings, is prepended to the provider request as a `system` message and is not inserted into the persisted chat transcript by this endpoint.
+- `enabledTools` limits Sybil-managed tools for this request. When omitted for a saved chat, the stored chat setting is used; otherwise all available tools are enabled by default. An empty array disables Sybil-managed tools.
 - Attachments are optional and are persisted under `message.metadata.attachments` on stored user messages when `persist` is `true`.
 
 Persisted chat streams with a `chatId` are backend-owned active runs: