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_api.py

@@ -34,9 +34,18 @@ from nanobot.webui.workspaces import (
     write_webui_default_access_mode,
 )
 
+from nanobot import __version__
+
 QueryParams = dict[str, list[str]]
 RuntimeSurface = Literal["browser", "native"]
 
+
+def _version_payload() -> dict[str, Any]:
+    """Return version info for the settings payload."""
+    return {
+        "current": __version__,
+    }
+
 _RUNTIME_CAPABILITIES = {
     "can_restart_engine": False,
     "can_pick_folder": False,
@@ -805,6 +814,7 @@ def settings_payload(
             "exec_path_append_set": bool(exec_config.path_append),
         },
         "requires_restart": requires_restart,
+        "version": _version_payload(),
     }
     return decorate_settings_payload(
         payload,