feat: add deepseek support, settings access, and model persistence

[Joshua Farrow (warrofua)]

Summary

• add DeepSeek models/provider support with reasoning_content passthrough for tool calls (per DeepSeek thinking-mode docs)
• expose Settings dialog in the UI, add API key management and auto-approve toggle
• persist selected model as the global default and restore it on startup

Settings UI access point:

Settings menu w/ auto-approve toggle:

Deepseek integration w/ model selection in UI:

Testing

• npm run typecheck
• npm run lint (repo-wide existing lint errors unrelated to this change)
• manual: verified DeepSeek selection + Reasoner, settings dialog opens, API key save, auto-approve toggle works, model selection persists across restart

[Joshua Farrow (warrofua)]

Closing in favor of clean DeepSeek-only PR: #39