ShopSettings.razor: new "AI Assistant" section with:
- Enable/disable toggle
- Provider dropdown (OpenAI / OpenRouter / Claude)
- Model input with per-provider placeholder
- API Key input (password with show/hide toggle)
- Per-provider hint text for getting API keys
- System Prompt textarea (optional, default used if empty)
- Save button with success/error feedback
- Config loads on init, saves via BFF PUT /api/bff/ai/config
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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