feat(wallpaper): support random cycling order

[hthienloc]

Description

This PR introduces a Random Order option for automatic wallpaper cycling. Currently, wallpapers in the active folder are strictly cycled in alphabetical order. With this change, users can enable random cycling so that the next wallpaper is selected at random, providing a surprise element.

How tested

• Verified that the "Random Order" switch displays in the settings under "Automatic Cycling".
• Verified that with random cycling enabled, wallpapers change in non-alphabetical random order and avoid duplicating the active image.

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that changes existing behavior)
• Refactor / internal cleanup
• Documentation
• Other

Related issues

Closes #1085

Screenshots / video

Checklist

• My code follows the conventions in CONTRIBUTING.md
• I have tested my changes locally
• New user-facing strings are wrapped in I18n.tr() with translator context, reusing existing terms where possible
• Go changes: ran make fmt, added/updated tests, make test passes, and go mod tidy is clean
• QML changes: ran make lint-qml with no new warnings
• I have opened a corresponding pull request in dlx-docs to document any new behaviors: https://github.com/AvengeMedia/DankLinux-Docs