- Add platform detection (Electron vs Web) for expand behavior
- Electron: 3-state (normal → 50% → 80%) with double-click
- Web/Mobile: 2-state (normal → expanded) with instant single tap
- Implement fixed 15-line height for web/mobile (360px, capped)
- Add orientation-aware height calculation (landscape vs portrait)
- Remove tooltip on web/mobile, keep for Electron desktop
- Add responsive placeholder text to prevent overlap on mobile
- Desktop: "Type your message, @file, @agent, or paste images and text..."
- Mobile (≤640px): "Type message, @file, @agent..."
- Delete dev-docs/expand-chat-input.md per upstream feedback
Addresses PR feedback to simplify from 3-state to 2-state for web/mobile
while maintaining rich desktop experience in Electron app.
