feat: add wallpaper picker popup with script and hyprpaper support

[defliez]

I hacked together a wallpaper picker in the taskbar :)
Supports hyprpaper and user-defined scripts

Features

• Thumbnail scroller that lists wallpapers from settings.wallpaperDir
• Clicking a wallpaper applies it:
  • If wallpaperScriptPath is set in settings.json, that script is used
  • Otherwise falls back to hyprpaper with a regenerated config
• Current wallpaper is displayed in the info row

Implementation

• Added WallpaperMenu.qml (new popup)
• Integrated in Bar.qml with a new taskbar button
• Extended Config.qml and settings.json with:
  • wallpaperDir
  • currentWallpaper
  • wallpaperScriptPath

[code-apeman]

To be fair, this (and the theme picker) belongs more to the settings window IMO

[code-apeman]

OK, so there were a couple of issues with the popup:

• The hardcoded /bin/bash path is incompatible with NixOS, as the only binaries stored in their default FHS locations are /bin/sh and /usr/bin/env
• As of right now, the hyprpaper syntax has changed

So, my suggestion is to change WallpaperMenu.qml:

176,177c176
<         printf 'preload = %s\n' "$wp" > "$cfg"
<         printf 'wallpaper = ,%s\n' "$wp" >> "$cfg"
---
> 	printf 'wallpaper {\n  monitor = \n  path = %s\n}' "$wp" > "$cfg"
182c181
<         startCmd(["/bin/bash", "-lc", sh, "qs", filePath]);
---
>         startCmd(["/bin/sh", "-lc", sh, "qs", filePath]);  // no such thing as a /bin/bash in nixos
188c187
<         startCmd(["/bin/bash", sp, filePath]);
---
>         startCmd(["/bin/sh", sp, filePath]);