Require user approval before downloading Agent updates

[mondalaci]

Summary

• Show a top notification bar when an Agent update is available, with Review changelog and Update actions, instead of auto-downloading in the background
• Rename the startup setting to Check for new Agent version on startup
• Let Check for update restore a previously dismissed notification bar, and fix the settings spinner staying visible after an update is found

Closes #2465

Test plan

• Enable Check for new Agent version on startup, restart Agent, and verify the update bar appears when a newer version exists
• Dismiss the bar with ×, click Check for update in Settings, and verify the bar reappears
• Click Review changelog and confirm it opens the GitHub CHANGELOG in the browser
• Click Update and confirm the update downloads and Agent restarts
• Verify the Check for update spinner stops after the check completes (whether or not an update is available)
• Verify forced Agent update (newer keyboard config) still auto-downloads and installs

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