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

package.json

@@ -10,6 +10,15 @@
     "build": "electron-vite build",
     "typecheck": "tsc --noEmit && tsc --noEmit -p tsconfig.node.json",
     "preview": "electron-vite preview",
+    "build:binaries": "bun scripts/build.ts",
+    "build:mac": "bun scripts/build.ts mac",
+    "build:mac-x64": "bun scripts/build.ts mac-x64",
+    "build:mac-arm64": "bun scripts/build.ts mac-arm64",
+    "build:win": "bun scripts/build.ts win",
+    "build:win-arm64": "bun scripts/build.ts win-arm64",
+    "build:linux": "bun scripts/build.ts linux",
+    "build:linux-arm64": "bun scripts/build.ts linux-arm64",
+    "build:all": "bun scripts/build.ts all",
     "package:mac": "electron-builder --mac",
     "package:win": "electron-builder --win",
     "package:linux": "electron-builder --linux"
@@ -45,6 +54,97 @@
     "files": [
       "dist/**/*",
       "package.json"
-    ]
+    ],
+    "mac": {
+      "category": "public.app-category.developer-tools",
+      "target": [
+        {
+          "target": "dmg",
+          "arch": [
+            "x64",
+            "arm64",
+            "universal"
+          ]
+        },
+        {
+          "target": "zip",
+          "arch": [
+            "x64",
+            "arm64",
+            "universal"
+          ]
+        }
+      ],
+      "artifactName": "${productName}-${version}-${os}-${arch}.${ext}",
+      "icon": "electron/resources/icon.icns"
+    },
+    "dmg": {
+      "contents": [
+        {
+          "x": 130,
+          "y": 220
+        },
+        {
+          "x": 410,
+          "y": 220,
+          "type": "link",
+          "path": "/Applications"
+        }
+      ]
+    },
+    "win": {
+      "target": [
+        {
+          "target": "nsis",
+          "arch": [
+            "x64",
+            "arm64"
+          ]
+        },
+        {
+          "target": "zip",
+          "arch": [
+            "x64",
+            "arm64"
+          ]
+        }
+      ],
+      "artifactName": "${productName}-${version}-${os}-${arch}.${ext}",
+      "icon": "electron/resources/icon.ico"
+    },
+    "nsis": {
+      "oneClick": false,
+      "allowToChangeInstallationDirectory": true,
+      "createDesktopShortcut": true,
+      "createStartMenuShortcut": true
+    },
+    "linux": {
+      "target": [
+        {
+          "target": "AppImage",
+          "arch": [
+            "x64",
+            "arm64"
+          ]
+        },
+        {
+          "target": "deb",
+          "arch": [
+            "x64",
+            "arm64"
+          ]
+        },
+        {
+          "target": "tar.gz",
+          "arch": [
+            "x64",
+            "arm64"
+          ]
+        }
+      ],
+      "artifactName": "${productName}-${version}-${os}-${arch}.${ext}",
+      "category": "Development",
+      "icon": "electron/resources/icon.png"
+    }
   }
 }