feat: add Linux build support (AppImage, deb, rpm)

Add Linux as a supported platform with AppImage, deb, and rpm targets
for both x64 and arm64 architectures.

Changes:
- electron-builder.yml: add linux section with AppImage/deb/rpm targets,
  icon, category, and desktop entry configuration
- package.json: add electron:pack:linux script
- electron/main.ts: fix getIconPath() to return .png on Linux instead
  of .icns (macOS format); enable loadUserShellEnv() on Linux since GUI
  apps launched from desktop also have limited env
- README.md + README_CN.md: update supported platforms section

The README previously stated 'Linux builds are planned. Contributions
welcome.' — this delivers on that.

package.json

@@ -12,7 +12,8 @@
     "electron:build": "next build && node scripts/build-electron.mjs",
     "electron:pack": "npm run electron:build && electron-builder --config electron-builder.yml",
     "electron:pack:mac": "npm run electron:build && electron-builder --mac --config electron-builder.yml",
-    "electron:pack:win": "npm run electron:build && electron-builder --win --config electron-builder.yml"
+    "electron:pack:win": "npm run electron:build && electron-builder --win --config electron-builder.yml",
+    "electron:pack:linux": "npm run electron:build && electron-builder --linux --config electron-builder.yml"
   },
   "dependencies": {
     "@anthropic-ai/claude-agent-sdk": "^0.2.33",