fix: auto-recover from offline state after reconnect

[ncthuc2004]

Summary

• add online/offline connection listeners so the extension can react immediately when network state changes
• treat API network error as an offline condition and render the offline warning badge instead of a generic error
• keep alarm-based polling active during offline periods so automatic recovery continues without manual icon clicks
• add tests for connection listeners and network-error classification

Why

Issue #132 reports that the extension can stay in an offline state until the user manually clicks the extension icon. This patch adds explicit connection-state handling and network-error fallback logic to recover automatically once connectivity returns.

Validation

• npm run test:js (passing)
• note: npm run lint:js fails in this environment due repository lint tooling compatibility (util.isDate in eslint-plugin-ava), unrelated to this patch logic

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IssueHunt Summary

Referenced issues

This pull request has been submitted to:

• #132: Notifier stays offline despite regaining connection

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