fix(firetv): unify not-awake error guidance

[huyua9]

Summary

• use one actionable message when a Fire TV remains asleep after retries
• apply the same guidance to EnsureAwake, Spotify startup, and app launch
• add a focused unit test for the shared message

Validation

• cargo fmt --check
• cargo test -p desk_remote_core firetv
• git diff --check

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