fix(bindings): reject empty LaunchApp package names

[xuyua9]

Summary

• reject empty or whitespace-only LaunchApp package names before persistence
• add a regression test proving invalid bindings are not written

Context

The binding store validates labels and hotkeys but currently accepts an incomplete LaunchApp action that cannot execute because it has no package identifier.

Validation

• cargo fmt --all -- --check
• cargo test -p desk_remote_core bindings
• git diff --check

[vercel]

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