fix: only show changelog after update, not on first install

[devin-ai-integration]

Summary

Fixes the changelog tab appearing when the app is first opened after download. The changelog should only show after an actual update, not on first install.

The bug was in maybe_emit_updated() - when there's no stored last_seen_version (first install), it was emitting the updated event with previous: None, which caused the changelog tab to open. Changed this to not emit the event on first install, while still saving the current version for future update detection.

Review & Testing Checklist for Human

• Test first install scenario: clear app data/use fresh install, verify changelog tab does NOT appear
• Test update scenario: install older version, update to newer version, verify changelog tab DOES appear with correct previous/current versions
• Verify the current version is still saved on first install (so subsequent updates will be detected)

Notes

Requested by @ComputelessComputer

Link to Devin run: https://app.devin.ai/sessions/ea84fde78ae047e2a21c0b7df4beacd3

[devin-ai-integration]

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