feat(wallpaper): add folder-based wallpaper selection and shuffle

[greenbugx]

Description

Adds a dedicated folder picker in the Settings wallpaper tab, allowing users to explicitly select an entire directory for automatic cycling without needing to manually select a dummy file.

Additionally, introduces a new Shuffle toggle to the Automatic Cycling configuration. When enabled, the background service seamlessly pipes the dynamically queried wallpaper list to shuf instead of sort, ensuring a truly random wallpaper is picked on every cycle while immediately adapting to any external file changes in the directory (e.g. from Nix or sync tools).

Note: Depends on the dank-qml-common FileBrowser folderMode fix submitted in this PR to resolve folder highlighting edge cases.

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 #2833

Screenshots / video

recording.mp4

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

[bbedward]

I merged the qml-common change, so you can bump the submodule ref in this PR