feat(download): add offline network indicator

[prince-0408]

Closes #637

Summary

Prevents silent failures on the download screen by detecting offline state before any network action is attempted, and surfacing a clear toast to the user.

Problem & Solution

Area	Before	After
Download button (offline)	Fails silently or generic error	Toast: "No internet connection"
"Check for new data" spinner (offline)	Hangs or fails silently	Toast: "No internet connection"
Network detection	None	NWPathMonitor singleton, passive background tracking

Changes

File	Type	Description
NetworkMonitor.swift	New	Lightweight NWPathMonitor singleton
DownloadStateManager.swift	Modified	Offline guards in handleDownloadAction & checkAllForUpdates

Notes

• Purely additive — zero existing logic changed or removed
• Reuses existing ToastView / ToastModifier infrastructure
• NWPathMonitor is Apple-native — no new dependencies

Screenshot

Toast shown when tapping "Update all", "Download data", or "Check for new data" while offline.

[github-actions]

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[github-actions]

Maintainer Checklist

The following is a checklist for maintainers to make sure this process goes as well as possible. Feel free to address the points below yourself in further commits if you realize that actions are needed :)

• Tests for changes have been written and the CI unit test, linting and formatting workflows within the PR checks do not indicate new errors in the files changed

• The CHANGELOG has been updated with a description of the changes for the upcoming release and the corresponding issue (if necessary)