Add Gemini as a chat model option

[arjunkshah]

Summary

• add Gemini Flash as a selectable model in the Clicky panel
• route /chat requests to Anthropic or Gemini based on the selected model id
• normalize Gemini streaming and non-streaming responses to the existing app response shape
• document the new GEMINI_API_KEY Worker secret

Testing

• DEVELOPER_DIR=/Applications/Xcode.app/Contents/Developer xcrun swiftc -parse leanring-buddy/AIChatAPI.swift leanring-buddy/CompanionManager.swift leanring-buddy/CompanionPanelView.swift
• npx -y -p typescript tsc --noEmit --target ES2022 --lib ES2022,WebWorker worker/src/index.ts
• git diff --check
• ran the Worker locally and verified gemini-2.5-flash returns normalized streaming content_block_delta events from /chat