feat: implement platform-specific expand chat input with mobile optimizations

- 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.

packages/opencode-config/package.json

@@ -3,6 +3,6 @@
   "version": "0.5.0",
   "private": true,
   "dependencies": {
-    "@opencode-ai/plugin": "1.1.10"
+    "@opencode-ai/plugin": "1.1.12"
   }
 }