feat: implement static loading placeholders for category sidebar

[pushtikadia]

… state closes #319

[ProdigyV21]

Thanks for this, the idea is good, but this PR is not ready yet.

After merging into current main, CategorySidebar.kt has broken leftover code around the new skeleton block, including }up, and the old SidebarRow arguments outside the block. That will not compile.

Also the title says animated skeleton, but right now it only adds static gray boxes. Please fix the structure so the original SidebarRow logic stays inside the else branch, remove the leftover comment/code, and either add real shimmer/animation or rename the PR to static loading placeholders.