feat(screen_recorder): add CPU encoder option

[AdityaKr015]

Summary

This PR adds a video_encoder setting (gpu/cpu, default gpu) to Screen Recorder. Selecting cpu passes -encoder cpu to gpu-screen-recorder, so recording works on GPUs with no hardware video encoder. Because the software encoder supports H.264 only, any other video_codec is forced to h264.

Motivation

For the system which have no hardware video encoding or just want to try cpu encoding.

Type of Change

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

Related Issue

Closes #45

Testing

• python3 validate-plugins.py = "Validated 13 plugin manifest(s)." all clean

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

Haven't tested yet

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

• Fix the issue after the review of the screen_recorder: add encoder setting with CPU fallback #49 instead of letting encoder = cpu + hevc/av1/vp8/vp9 fail at startup, the builders force h264.

[ItsLemmy]

Excellent, thanks!

[AdityaKr015]

Thanks for the merge