Fix repeated update banner notifications

[SatorSK]

Fixes PlatformNetwork/bounty-challenge#53190.

Summary

• Use last_notified_version for startup update notifications
• Suppress repeat banners for the same latest version
• Keep regular check_update() behavior unchanged for explicit update/install flows
• Add focused config coverage for notification tracking

Why

The update config already had last_notified_version, but the startup update check never consulted or persisted it. This meant the same available-update banner could reappear on every launch. The new notification-specific check records the version after surfacing it, while explicit update checks still report available updates normally.

Verification

• rustfmt --edition 2024 --check src\\cortex-update\\src\\config.rs src\\cortex-update\\src\\manager.rs src\\cortex-tui\\src\\runner\\app_runner\\runner.rs
• cargo check -p cortex-tui
• cargo test -p cortex-update --lib compiled but the Windows test executable could not run because it requires elevation (os error 740)

[greptile-apps]

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