settings: static llm.provider options list is stale (missing all providers added after requesty)

Follow-up from PR #6578.

The static options list for llm.provider in defaults/default_settings.json ends at requesty — it is missing orcarouter, atlascloud, qwen, xai, ionos, and deepseek. Two consumers notice:

1. static/js/utils/provider-options.js uses it only as the fallback when the backend available-models call fails (offline/degraded) — the degraded dropdown then hides those providers entirely.
2. #6486's settings-page relevance filter (settings.js) derives provider prefixes from these options — with a provider missing from the list, its llm.<p>.* settings are never hidden as irrelevant for other providers.

Suggested: regenerate the list from auto-discovery at build time, or prune it entirely so the fallback and filter degrade uniformly.