fix: highlight device login copy action

[SlaVKsVolks]

Summary

• promotes the device-login c shortcut to a primary action-bar item
• keeps Enter as an alternate copy hint without merging it into the same dim Enter/c label
• adds coverage that pending login key hints expose Enter and c separately

Fixes PlatformNetwork/bounty-challenge#53260.

Verification

• rustfmt --edition 2024 --check src/cortex-tui/src/modal/login.rs
• cargo test -p cortex-tui modal::login::tests::test_pending_key_hints_split_copy_keys
• cargo test -p cortex-tui modal::login::tests::test_key_handling_pending
• cargo check -p cortex-tui
• git diff --check

[greptile-apps]

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