feat: add custom/other OpenAI-compatible LLM providers

Allow Settings → Providers to define custom providers (id, name, base URL,
API key, models, headers) without code changes. Persist config via OpenCode
layers, store keys through auth.set, and keep web/VS Code parity.

Co-authored-by: Serhii Dziupin <makeittech@users.noreply.github.com>

packages/ui/src/lib/settings/search.ts

@@ -754,6 +754,13 @@ const SETTINGS_SEARCH_ITEMS: readonly SettingsSearchItem[] = [
     titleKey: 'settings.providers.page.connect.title',
     keywords: ['add provider', 'connect provider', 'credentials'],
   },
+  {
+    id: 'providers.custom',
+    page: 'providers',
+    titleKey: 'settings.providers.page.custom.title',
+    descriptionKey: 'settings.providers.page.custom.description',
+    keywords: ['other', 'custom', 'openai-compatible', 'base url', 'api key'],
+  },
   {
     id: 'providers.auth',
     page: 'providers',