fix(tui): polish background tasks empty state copy

[ChuanWeis]

Summary

• Update the background tasks empty-state copy to the grammatical sentence No background tasks are running.
• Move the empty-state copy into a small constant and add a focused regression test for the expected wording

Refs PlatformNetwork/bounty-challenge#53146.

Verification

• git diff --check
• Searched the TUI views for the old wording and confirmed it is no longer present

I could not run cargo fmt or cargo test locally because this Windows environment does not currently have cargo available on PATH.

[greptile-apps]

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[ChuanWeis]

Update: I installed the Rust toolchain locally and reran the targeted validation for this branch.

• cargo test -p cortex-tui test_empty_tasks_message_is_grammatical passes.