fix: remove unused columns property in FilterMenuView

The columns property was leftover from the initial grid layout design
and is no longer needed after switching to vertical list layout.

Addresses Copilot review comment.

Author: graycreate

V2er/View/Feed/FilterMenuView.swift

@@ -15,12 +15,6 @@ struct FilterMenuView: View {
     let onTabSelected: (Tab) -> Void
     let onDismiss: () -> Void
 
-    private let columns = [
-        GridItem(.flexible()),
-        GridItem(.flexible()),
-        GridItem(.flexible())
-    ]
-
     var body: some View {
         ZStack(alignment: .topLeading) {
             if isShowing {