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.

electron/main.ts

@@ -71,12 +71,13 @@ function checkNativeModuleABI(): void {
 
 /**
  * Read the user's full shell environment by running a login shell.
- * When Electron is launched from Dock/Finder, process.env is very limited
- * and won't include vars from .zshrc/.bashrc (e.g. API keys).
+ * When Electron is launched from Dock/Finder (macOS) or desktop launcher
+ * (Linux), process.env is very limited and won't include vars from
+ * .zshrc/.bashrc (e.g. API keys, nvm PATH).
  */
 function loadUserShellEnv(): Record<string, string> {
-  // Only macOS needs login-shell env loading; Windows/Linux GUI apps inherit full env
-  if (process.platform !== 'darwin') {
+  // Windows GUI apps inherit the full user environment
+  if (process.platform === 'win32') {
     return {};
   }
   try {
@@ -250,6 +251,9 @@ function getIconPath(): string {
   if (process.platform === 'win32') {
     return path.join(process.resourcesPath, 'icon.ico');
   }
+  if (process.platform === 'linux') {
+    return path.join(process.resourcesPath, 'icon.png');
+  }
   return path.join(process.resourcesPath, 'icon.icns');
 }