refactor(webui): replace real-time polling with click-to-check version updates

- Remove background PyPI polling loop and WebSocket broadcast
- Remove UpdateBanner from ThreadHeader (keep main page clean)
- Add on-demand version check endpoint (GET /api/settings/version-check)
- Add 'About' section in Settings > Overview with check-for-updates button
- Design: no auto-fetch, user initiates check explicitly via button click

nanobot/webui/settings_routes.py

@@ -36,6 +36,7 @@ from nanobot.webui.settings_api import (
     update_transcription_settings,
     update_web_search_settings,
 )
+from nanobot.webui.version_check import check_for_update
 
 QueryParams = dict[str, list[str]]
 
@@ -117,6 +118,8 @@ class WebUISettingsRouter:
             return await self._handle_settings_cli_apps_action(request, "test")
         if path == "/api/settings/mcp-presets":
             return await self._handle_settings_mcp_presets(request)
+        if path == "/api/settings/version-check":
+            return await self._handle_settings_version_check(request)
         mcp_action = _MCP_PRESET_ACTIONS_BY_PATH.get(path)
         if mcp_action is not None:
             return await self._handle_settings_mcp_presets(request, mcp_action)
@@ -347,3 +350,15 @@ class WebUISettingsRouter:
         if action is None:
             return self._json_response(payload)
         return self._json_response(self._with_restart_state(payload, section="runtime"))
+
+    async def _handle_settings_version_check(self, request: WsRequest) -> Response:
+        if not self._authorized(request):
+            return self._unauthorized()
+        try:
+            update_info = await asyncio.to_thread(check_for_update)
+        except Exception:
+            self.logger.exception("version check failed")
+            return self._error_response(500, "version check failed")
+        return self._json_response({
+            "updateAvailable": update_info,
+        })