feat(bongocat): add executable_path setting

[NousadS]

Summary

Added executable_path setting, replacing the previous hardcoded executable path.

Motivation

See #4.

Type of Change

• Bug fix
• New feature
• Breaking change
• Refactoring
• Build / packaging

Related Issue

Closes #4.

Testing

Tested the executable_path setting with:

• Valid paths: evtest, /usr/bin/evtest, /run/current-system/sw/bin/evtest
• Invalid paths: /path, /path/, /usr/, /

(Note: Directory paths like / and /usr/ should give an error but don't do so, see noctalia#3167)

Manual Coverage

• Tested on Niri
• Tested on Hyprland
• Tested on Sway
• Tested on another compositor:
• Tested with different bar positions and density settings
• Tested at different interface scaling values
• Tested with multiple monitors

Screenshots / Videos

Checklist

• This PR is ready for review, or it is marked as Draft.
• I self-reviewed the changes.
• I added or updated translations/en.json, or this PR adds no new user-facing strings.
• I did not edit non-English translation files unless this PR is explicitly for translation tooling, an import/export sync, or a maintainer-requested locale change.

Additional Notes