Use user-selected accent color in Home, Subscriptions, and Downloads

Color.accentColor and .foregroundStyle(.tint) resolve to the asset
catalog accent on macOS, so Home shortcut cards, section header
links, the Subscriptions "All channels" header, and the Downloads
per-channel group headers stayed blue when the user picked a
different accent. Read the color from SettingsManager and apply it
directly, matching the pattern already used for the Play button.

Yattee/Views/Downloads/DownloadsView.swift

@@ -347,6 +347,10 @@ private struct CompletedDownloadsSectionContentView: View {
     let listStyle: VideoListStyle
     let isGroupedMode: Bool
 
+    private var accentColor: Color {
+        appEnvironment?.settingsManager.accentColor.color ?? .accentColor
+    }
+
     private var completedFiltered: [Download] {
         guard !searchText.isEmpty else { return manager.completedDownloads }
         let query = searchText.lowercased()
@@ -502,7 +506,7 @@ private struct CompletedDownloadsSectionContentView: View {
                 Image(systemName: "chevron.right")
                     .font(.caption)
             }
-            .foregroundStyle(Color.accentColor)
+            .foregroundStyle(accentColor)
         }
         .zoomTransitionSource(id: channelID)
         .buttonStyle(.plain)