feat: add user setting for update notification interval

[KaixunWang]

Summary

Closes #2597

Adds a user setting to control how frequently the update-ready dialog is shown.

Changes

• Added updateNotificationInterval setting with options: DAILY / WEEKLY / MONTHLY / NEVER
• Added dropdown UI in Settings page
• Added throttle logic in src/main/updater.ts: suppresses showUpdateReadyDialog if interval hasn't elapsed since last prompt
• Tray menu "Restart to install" still appears immediately regardless of setting

Behavior

• NEVER: update downloads silently, no dialog shown
• DAILY/WEEKLY/MONTHLY: dialog only shown if enough time has passed since last prompt
• Default is DAILY to preserve existing behavior
• Note: The update is always downloaded in the background regardless of the setting. Only the dialog prompt is throttled. The tray menu "Restart to install" appears immediately once download completes.(I think that's what the issue was talking about)

[setchy]

Thanks for the contribution @KaixunWang, greatly appreciated.

I'm still thinking through whether we would want to support this feature in Gitify.

I understand we don't want to create too much noise for end users, but, as the core maintainer, I also want to make sure the user-base keeps pace with the project as it evolves and as features/fixes are added.

Looking for additional input from the user-base and other maintainers first before considering this addition

[KaixunWang]

Thanks for the feedback! Totally understand. Happy to wait for input from the community and other maintainers before moving forward. Let me know if there's anything you'd like me to adjust in the meantime.

Also, just want to say Gitify is a fantastic tool, it's super useful. Great work maintaining it!