Fix responsive width for laptop screens

Restore max-w-3xl as default width, only widen on larger screens:
- Default: max-w-3xl (768px) for laptops
- xl (1280px+): max-w-5xl for external monitors
- 2xl (1536px+): max-w-6xl for large desktops

Bumps version to 0.4.3.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/opencode-plugin/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@plannotator/opencode",
-  "version": "0.4.2",
+  "version": "0.4.3",
   "description": "Plannotator plugin for OpenCode - interactive plan review with visual annotation",
   "author": "backnotprop",
   "license": "BSL-1.1",