fix empty detail view on thread select, reduce toolbar overflow

- add .id(selectedThread?.id) to ThreadDetailView to force SwiftUI
  NavigationSplitView detail re-evaluation on selection change
- selectMailbox clears selectedThread/messages to avoid stale state
- remove defer/project buttons from folder toolbar (4 buttons instead of 7),
  keyboard shortcuts still work

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Apps/MagnumOpus/ContentView.swift

@@ -63,6 +63,7 @@ struct ContentView: View {
 					requestCompose(mode)
 				}
 			)
+			.id(viewModel.selectedThread?.id)
 		}
 		.safeAreaInset(edge: .bottom) {
 			statusBanner

Apps/MagnumOpus/ViewModels/MailViewModel.swift

@@ -187,6 +187,8 @@ final class MailViewModel {
 	func selectMailbox(_ mailbox: MailboxInfo) {
 		selectedMailbox = mailbox
 		selectedItem = nil
+		selectedThread = nil
+		messages = []
 		items = []
 		guard let accountConfig else { return }
 		startObservingThreads(accountId: accountConfig.id, mailboxId: mailbox.id)

Apps/MagnumOpus/Views/ThreadListView.swift

@@ -161,31 +161,6 @@ struct ThreadListView: View {
 				}
 				.keyboardShortcut("u", modifiers: [.shift, .command])
 				.help("Toggle Read/Unread (⇧⌘U)")
-
-				Button {
-					showMoveSheet = true
-				} label: {
-					Label("Move", systemImage: "folder")
-				}
-				.keyboardShortcut("m", modifiers: [.shift, .command])
-				.help("Move to… (⇧⌘M)")
-
-				// GTD shortcuts available in folder views too
-				Button {
-					showDeferPicker = true
-				} label: {
-					Label("Defer", systemImage: "clock.arrow.circlepath")
-				}
-				.keyboardShortcut("d", modifiers: [])
-				.help("Defer (d)")
-
-				Button {
-					showLabelPicker = true
-				} label: {
-					Label("Project", systemImage: "folder.badge.plus")
-				}
-				.keyboardShortcut("p", modifiers: [])
-				.help("File to Project (p)")
 			}
 		}
 	}