fix: use PUBLIC_PATH environment variable in WebpackPwaManifest

- Replace hardcoded publicPath: './' with process.env.PUBLIC_PATH || 'auto'
- Ensures consistent path handling across all webpack plugins
- Allows PWA manifest to use correct base path for different deployments

Test confirmed: PUBLIC_PATH="/smalruby3-gui/" now correctly generates
<script defer src="/smalruby3-gui/gui.js"></script> in index.html

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: takaokouji

webpack.config.js

@@ -187,7 +187,7 @@ const buildWithPwaConfig = buildConfig.clone()
     )
     .addPlugin(
         new WebpackPwaManifest({
-            publicPath: './',
+            publicPath: process.env.PUBLIC_PATH || 'auto',
             name: 'Smalruby',
             short_name: 'Smalruby',
             description: 'GraphicaL User Interface for creating and running Smalruby 3.0 projects',