fix(cli): show search engines (incl. Keenable) in onboard wizard

The onboard wizard dispatched field handlers by bare field name, so
WebSearchConfig.provider was hijacked by the LLM-provider handler and
showed LLM providers instead of search engines. Keenable was also never
wired into the CLI wizard when it landed in the WebUI.

- Add a single source of truth for selectable search providers
  (SEARCH_PROVIDER_OPTIONS in web.py); WebUI settings now import it.
- Add a WebSearchConfig-aware search-provider picker to the wizard and
  resolve handlers by (model type, field name) so the LLM and search
  provider fields no longer collide.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/agent/test_onboard_logic.py

@@ -1998,3 +1998,27 @@ class TestModelPresetWizard:
         defaults = AgentDefaults()
         _handle_provider_field(defaults, "provider", "Provider", "auto")
         assert defaults.provider == "anthropic"
+
+    def test_search_provider_field_handler(self, monkeypatch):
+        """_handle_search_provider_field should set the search engine from choices."""
+        from nanobot.agent.tools.web import WebSearchConfig
+        from nanobot.cli.onboard import _handle_search_provider_field
+
+        monkeypatch.setattr(onboard_wizard, "_select_with_back", lambda *a, **kw: "keenable")
+
+        cfg = WebSearchConfig()
+        _handle_search_provider_field(cfg, "provider", "Provider", "duckduckgo")
+        assert cfg.provider == "keenable"
+
+    def test_provider_field_dispatch_is_model_type_aware(self):
+        """WebSearchConfig.provider must not be hijacked by the LLM provider handler."""
+        from nanobot.agent.tools.web import WebSearchConfig
+        from nanobot.cli.onboard import (
+            _handle_provider_field,
+            _handle_search_provider_field,
+            _resolve_field_handler,
+        )
+        from nanobot.config.schema import AgentDefaults
+
+        assert _resolve_field_handler(WebSearchConfig(), "provider") is _handle_search_provider_field
+        assert _resolve_field_handler(AgentDefaults(), "provider") is _handle_provider_field