fix(ios): clear the List row fill so a declared list background shows

[CodyPChristian]

The bug

ListBackgroundModifier hides the grouped scroll background when a list node declares one:

content
    .scrollContentBackground(.hidden)
    .background(Color(argb: argb))

But SwiftUI also gives every row an opaque system fill, and that paints over the list's background. So <list class="bg-theme-background"> still renders as a white sheet, with the declared colour visible only in the thin strips above the first row and below the last.

The effect is that a themed list-based screen looks broken next to an otherwise-identical scroll-view-based one.

The fix

One line in rowView():

.listRowBackground(Color.clear)

The row becomes transparent, so the list's declared background shows through, and each row still paints whatever its own content declares (a card, a surface colour, nothing).

Scope

• Only affects lists that declare a background — ListBackgroundModifier is already a no-op otherwise, so the stock grouped look is unchanged for everyone else.
• Android is unaffected: Compose has no implicit row fill, and ListRenderer.kt already draws MaterialTheme.colorScheme.background.

Verified

Built and run on an iOS 26 simulator against a themed app: a <list> screen now matches its <scroll-view> equivalent, rows still render their own card backgrounds, and separators/swipe actions are unchanged.