feat: application icon for Windows taskbar + exe

- scripts/gen_icon.py: generates 256x256 SSH client icon (dark terminal
  window with >_ prompt and SSH connection nodes), outputs multi-size
  ICO (16-256px) + PNG
- resources/icon.ico + icon.png: generated icon files
- main.py: _get_icon_path() searches _MEIPASS (onefile), exe dir, source
  dir; sets app + window icon
- build.spec: bundles icon files via datas=, sets exe icon=icon.ico
- Windows taskbar will now show the custom icon instead of default PyInstaller icon

build.spec

@@ -31,8 +31,8 @@ a = Analysis(
     pathex=[str(ROOT)],
     binaries=[],
     datas=[
-        # 如果后续要加资源文件（图标、配置模板），放这里
-        # ('resources/icon.ico', 'resources'),
+        ('resources/icon.png', 'resources'),
+        ('resources/icon.ico', 'resources'),
     ],
     hiddenimports=hiddenimports,
     hookspath=[],
@@ -80,5 +80,5 @@ exe = EXE(
     target_arch=None,
     codesign_identity=None,
     entitlements_file=None,
-    # icon='resources/icon.ico' if Path('resources/icon.ico').exists() else None,
+    icon='resources/icon.ico' if Path('resources/icon.ico').exists() else None,
 )