Add combine saved conversations feature (simple + AI-assisted merge)

Lets users multi-select 2+ saved conversations and merge them into one,
either by chronological concatenation or by having the default model
synthesize a single coherent conversation from the source transcripts.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

oAI/Views/Screens/ConversationListView.swift

@@ -36,6 +36,7 @@ struct ConversationListView: View {
     @State private var semanticResults: [Conversation] = []
     @State private var isSearching = false
     @State private var selectedIndex: Int = 0
+    @State private var showCombineSheet = false
     @FocusState private var searchFocused: Bool
     private let settings = SettingsService.shared
     var onLoad: ((Conversation) -> Void)?
@@ -70,6 +71,18 @@ struct ConversationListView: View {
                     }
                     .buttonStyle(.plain)
 
+                    if selectedConversations.count >= 2 {
+                        Button {
+                            showCombineSheet = true
+                        } label: {
+                            HStack(spacing: 4) {
+                                Image(systemName: "arrow.triangle.merge")
+                                Text("Combine (\(selectedConversations.count))")
+                            }
+                        }
+                        .buttonStyle(.plain)
+                    }
+
                     if !selectedConversations.isEmpty {
                         Button(role: .destructive) {
                             deleteSelected()
@@ -298,6 +311,16 @@ struct ConversationListView: View {
             searchFocused = true
         }
         .frame(minWidth: 700, idealWidth: 800, minHeight: 500, idealHeight: 600)
+        .sheet(isPresented: $showCombineSheet) {
+            CombineConversationsSheet(
+                conversations: conversations.filter { selectedConversations.contains($0.id) },
+                onCompleted: { _ in
+                    loadConversations()
+                    selectedConversations.removeAll()
+                    isSelecting = false
+                }
+            )
+        }
     }
 
     private func loadConversations() {