Extract the disabled composer view into its own struct

ElementX/Sources/Other/SwiftUI/Views/ComposerDisabledView.swift

@@ -0,0 +1,19 @@
+//
+// Copyright 2025 New Vector Ltd.
+//
+// SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+// Please see LICENSE files in the repository root for full details.
+//
+
+import Compound
+import SwiftUI
+
+struct ComposerDisabledView: View {
+    var body: some View {
+        Text(L10n.screenRoomTimelineNoPermissionToPost)
+            .font(.compound.bodyLG)
+            .foregroundStyle(.compound.textDisabled)
+            .multilineTextAlignment(.center)
+            .padding(.vertical, 10) // Matches the MessageComposerStyleModifier
+    }
+}