Base Clip AI for everyone (host default)

Added

• A base model served to all signed-in users. The host env fallback (AI_API_KEY/OPENROUTER_API_KEY + AI_MODEL) is the base Clip AI: any authenticated user without their own model resolves to it, so basic AI works with no per-user key. Documented nvidia/nemotron-3-super-120b-a12b:free as a free OpenRouter base in .env.example, and added it to the composer's model switcher presets. "Bring your own model" still takes priority.