feat: add offline network indicator on download screen

Terms

• I have searched open and closed feature requests
• I agree to follow Scribe-iOS' Code of Conduct

Description

Summary

Currently, tapping "Download data", "Update all", or "Check for new data" while offline fails silently with no user feedback.

Proposed Solution

• Add a lightweight NetworkMonitor singleton using Apple's native NWPathMonitor
• Hook it into DownloadStateManager to guard download actions when offline
• Show a clear toast: "No internet connection. Please connect and try again."

Affected Areas

• handleDownloadAction
• checkAllForUpdates
• handleUpdateAllLanguages

No existing logic will be changed — purely additive.

Contribution

No response