OpenAI API support

[BrandeisPatrick]

Add optional OpenAI API support alongside local Ollama.

Requirements:

• Create OpenAIService.swift with streaming support
• Store API key securely in macOS Keychain
• Add settings UI for API endpoint and key
• Model dropdown shows both local and cloud options
• Privacy indicator (🔒 Local vs ☁️ Cloud)
• Support GPT-5-mini and other OpenAI models

Files to create/modify:

• OpenAIService.swift (new)
• ContentView.swift
• SettingsView.swift (new)