feat(gpt-apps): add web scaffold and placeholder widgets

[jakcinmarina]

Summary

This PR introduces the initial GPT Apps web scaffold, foundation for rendering GPT Apps widgets in the browser. Goal was to set up the project structure and shared UI components.

Changes

• Added initial web project setup under src/web
• Introduced build configuration for widget bundles
• Added shared UI and layout components
• Added minimal placeholder widget components to mimic the structure

Notes

• Widget components in this PR are intentionally placeholders (will be added in following PR)
• No real widget logic, data handling, or UX is implemented yet

Next steps

• Replace placeholder widgets with real widget/page implementations