feat(wallpaper): add current wallpaper to favorites

[AdityaKr015]

Summary

This PR add a star in wallpaper panel to adds current wallpaper to favorites.

Motivation

1 click to favorite the currently applied wallpaper including its color scheme and dark/light theme, its a good UX to have.

Type of Change

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

Related Issue

Closes #4018

Testing

• Ran just format, just build, just test everything was clean
• Then python3 tools/i18n-check.py
  OK: 893 tr keys, 7 trp base keys — all present in catalog (2540 entries).
• Killed current noctalia and starts build version noctalia
• Tested with switching to unfavorite wallpaper and using star button to favorite it and it worked

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

recording_20260820_201212.mp4

Checklist

• This PR is ready for review,.
• I read and followed the relevant guidance in CONTRIBUTING.md.
• I ran just format with clang-format v22+ installed.
• I ran the relevant build or test commands.
• I self-reviewed the changes.
• I checked for new warnings or errors.
• I will update end-user documentation after merge, or this PR does not change user-facing configuration or behavior.
• I added or updated assets/translations/en.json.
• 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.
• I used the existing canonical names for config keys, IPC names, paths, and identifiers.