fix: improve new-project network error diagnostics

[SergioChan]

Summary

Improve frontend error feedback when creating a new project so users can quickly diagnose "Network Error" and timeout failures instead of seeing a generic message.

Changes

• Added formatProjectInitError in frontend/src/views/Process.vue
• Distinguish timeout errors and provide actionable hint (reduce file size / check model speed)
• Distinguish network errors and show configured backend URL (VITE_API_BASE_URL fallback)
• Surface backend error/message payload when available

Testing

• Manual code-path review for handleNewProject catch block
• No automated frontend test harness in repo for this view

Fixes #121