Add List/Grid layout option for Home sections

Introduces a "Display sections as" picker in Home settings with List and
Grid modes. Grid renders each section as a horizontal shelf of video
cards, defaulting to Grid on tvOS and List on iOS/macOS. Per-platform
defaults are preserved via a platform-specific settings key.

On tvOS the shelf is a focus section so swiping up/down between rows of
different lengths works without getting stuck at the end of a row.

Yattee/Views/Home/HomeHorizontalCards.swift

@@ -0,0 +1,53 @@
+//
+//  HomeHorizontalCards.swift
+//  Yattee
+//
+//  Horizontal shelf of video cards for Home sections in grid layout mode.
+//
+
+import SwiftUI
+
+/// A horizontally scrolling row of `VideoCardView` cards used by Home sections
+/// when `HomeSectionLayout.grid` is selected.
+struct HomeHorizontalCards: View {
+    let videos: [Video]
+    let queueSource: QueueSource
+    let sourceLabel: String
+    var loadMoreVideos: LoadMoreVideosCallback? = nil
+
+    #if os(tvOS)
+    private let cardWidth: CGFloat = 320
+    private let cardHeight: CGFloat = 340
+    private let spacing: CGFloat = 60
+    private let verticalPadding: CGFloat = 28
+    #else
+    private let cardWidth: CGFloat = 180
+    private let cardHeight: CGFloat = 210
+    private let spacing: CGFloat = 28
+    private let verticalPadding: CGFloat = 8
+    #endif
+
+    var body: some View {
+        ScrollView(.horizontal, showsIndicators: false) {
+            LazyHStack(alignment: .top, spacing: spacing) {
+                ForEach(Array(videos.enumerated()), id: \.element.id) { index, video in
+                    VideoCardView(video: video, isCompact: true)
+                        .frame(width: cardWidth, height: cardHeight, alignment: .top)
+                        .tappableVideo(
+                            video,
+                            queueSource: queueSource,
+                            sourceLabel: sourceLabel,
+                            videoList: videos,
+                            videoIndex: index,
+                            loadMoreVideos: loadMoreVideos
+                        )
+                }
+            }
+            .padding(.horizontal)
+            .padding(.vertical, verticalPadding)
+            #if os(tvOS)
+            .focusSection()
+            #endif
+        }
+    }
+}