Add channels sidebar to Subscriptions on iPad regular width

On iPad in regular horizontal size class, the Subscriptions view now
shows a channels column next to the feed (mirroring macOS/tvOS) instead
of the floating channel strip. iPhone and compact-width iPad keep the
existing strip.

- Renames MacSubscriptionsSidebarRow to SubscriptionsSidebarRow and
  shares it across macOS and iOS.
- Uses a custom ScrollView-based sidebar on iPad to avoid iOS 26's
  sidebar background extension bleeding into the selection highlight.
- Forces inline toolbar title on iPad regular so scrolling either
  column behaves consistently.

Yattee/Views/Subscriptions/SubscriptionsSidebarRow.swift

@@ -0,0 +1,77 @@
+//
+//  SubscriptionsSidebarRow.swift
+//  Yattee
+//
+//  Compact row used in the Subscriptions sidebar (macOS and iPad) for the
+//  "All Channels" entry and each subscribed channel.
+//
+
+import SwiftUI
+import NukeUI
+
+struct SubscriptionsSidebarRow: View {
+    let name: String
+    let avatarURL: URL?
+    let serverURL: URL?
+    let authHeader: String?
+    let channelID: String?
+    let isAllChannels: Bool
+    var isSelected: Bool = false
+
+    private let avatarSize: CGFloat = 28
+
+    private var effectiveAvatarURL: URL? {
+        guard let channelID else { return nil }
+        return AvatarURLBuilder.avatarURL(
+            channelID: channelID,
+            directURL: avatarURL,
+            serverURL: serverURL,
+            size: Int(avatarSize * 2)
+        )
+    }
+
+    var body: some View {
+        HStack(spacing: 8) {
+            avatar
+                .frame(width: avatarSize, height: avatarSize)
+                .clipShape(Circle())
+
+            Text(name)
+                .fontWeight(isSelected ? .semibold : .regular)
+                .lineLimit(1)
+
+            Spacer(minLength: 0)
+        }
+        .contentShape(Rectangle())
+    }
+
+    @ViewBuilder
+    private var avatar: some View {
+        if isAllChannels {
+            ZStack {
+                Circle().fill(.quaternary)
+                Image(systemName: "rectangle.stack.fill")
+                    .resizable()
+                    .aspectRatio(contentMode: .fit)
+                    .foregroundStyle(.secondary)
+                    .padding(avatarSize * 0.25)
+            }
+        } else {
+            LazyImage(request: AvatarURLBuilder.imageRequest(url: effectiveAvatarURL, authHeader: authHeader)) { state in
+                if let image = state.image {
+                    image
+                        .resizable()
+                        .aspectRatio(contentMode: .fill)
+                } else {
+                    Circle()
+                        .fill(.quaternary)
+                        .overlay {
+                            Text(String(name.prefix(1)))
+                                .font(.caption)
+                                .foregroundStyle(.secondary)
+                        }
+                }
+            }
+        }
+    }
+}