feat(ui): redesign AI Provider settings with provider presets and model catalog

- Replace hardcoded provider list with PROVIDER_PRESETS (OpenAI, Ollama,
  DeepSeek, Groq, OpenRouter, OpenCode Go, Custom)
- Load model lists from models.dev/api.json catalog at startup
- Add Combobox vanilla JS component for model/base-URL selection
- Fetch local Ollama models via live API instead of catalog
- Hide API key values from frontend (boolean-only llm_api_key_set)
- Add i18n translations for all 9+ locales
- Update snapshot tests for new response fields

tests/routes/__snapshots__/test_api_snapshots.ambr

@@ -28,6 +28,7 @@
     'settings': dict({
       'civitai_api_key_set': True,
       'language': 'en',
+      'llm_api_key_set': False,
       'theme': 'dark',
     }),
     'success': True,