Add binary build system for cross-platform distribution

- Add scripts/build.ts: Bun-based build script for generating binaries
- Support macOS (x64, ARM64, Universal), Windows (x64, ARM64), Linux (x64, ARM64)
- Add build:* npm scripts for each platform (build:mac, build:win, build:linux, etc)
- Configure electron-builder with platform-specific settings:
  * macOS: DMG + ZIP, Universal binaries, proper icon paths
  * Windows: NSIS installer + ZIP, configurable install directory
  * Linux: AppImage + DEB + tar.gz packages
- Add BUILD.md: Comprehensive build documentation with examples
- Update README.md: Add build instructions and reference BUILD.md
- Artifacts named: OpenCode Client-{version}-{os}-{arch}.{ext}
- Output directory: release/

Usage:
  bun run build:mac       # macOS Universal (Intel + Apple Silicon)
  bun run build:win       # Windows x64
  bun run build:linux     # Linux x64
  bun run build:all       # All platforms

README.md

@@ -65,10 +65,13 @@ bun run dev
 # Build for production
 bun run build
 
-# Package for distribution
-bun run package:mac    # macOS
-bun run package:win    # Windows
-bun run package:linux  # Linux
+# Build distributable binaries
+bun run build:mac      # macOS (Universal)
+bun run build:win      # Windows (x64)
+bun run build:linux    # Linux (x64)
+bun run build:all      # All platforms
+
+# See BUILD.md for more build options
 ```
 
 ## Development