Settings polish: /setup provider subs, Add API defaults to api kind, picker shows offline endpoints, doc library tracks sub-tab

- /setup gains explicit provider subcommands (deepseek, openai,
  anthropic, openrouter, groq, gemini, xai, ollama, copilot, local,
  endpoint) so the autocomplete popup surfaces "/setup de…" suggestions
  with format hints, and bare-provider invocations still prompt for
  the key.
- Add API endpoint defaults to kind=api (auto-refresh /v1/models)
  instead of kind=proxy. Proxy was a frequent footgun for OpenAI-
  compatible endpoints that DO serve /v1/models — the user got an
  empty model list and had to flip the dropdown.
- Model picker now includes offline endpoints with stale:true so a
  briefly-down local server doesn't vanish from the picker (it dims
  and shows the offline pill, clickable anyway). Dedup prefers the
  online entry when the same model is exposed by both.
- Document library modal header reflects the active sub-tab via
  _TAB_HEADERS so it no longer shows the wrong section name when
  switching between Documents / Skills / Templates.

static/js/admin.js

@@ -731,14 +731,14 @@ function initEndpointForm() {
   urlInput.addEventListener('input', () => {
     if (provider.value && urlInput.value.trim() !== provider.value) {
       provider.value = '';
-      if (kindSel) kindSel.value = 'proxy';
+      if (kindSel) kindSel.value = 'api';
       _renderPickerMenu();
       _syncPickerCurrent();
     }
   });
-  if (kindSel) kindSel.value = provider.value ? 'api' : (kindSel.value || 'proxy');
+  if (kindSel) kindSel.value = kindSel.value || 'api';
   function _apiEndpointKind() {
-    return (kindSel && kindSel.value) ? kindSel.value : (provider.value ? 'api' : 'proxy');
+    return (kindSel && kindSel.value) ? kindSel.value : 'api';
   }
   function _normalizeBaseUrl(raw) {
     let u = raw.trim();