Fix deletions not sticking (git sync resurrection), persist folder

collapse state, make merge provider picker visibly clickable

Deleted conversations coming back: exportAllConversations() only ever
wrote files for conversations that currently exist — it never removed
the exported markdown file for a conversation that had been deleted
locally. That file just sits in the sync repo forever, so every
future pull+import (including on every app startup) silently
resurrects it, since importAllConversations() only skips an import
when a matching local ID already exists. Fixed by having export also
delete orphaned files (conversation ID no longer present locally), and
added GitSyncService.syncAfterDeletion() — a debounced export+push
triggered right after any delete/bulk-delete/merge-cleanup, so the
removal reaches the remote promptly instead of waiting on an
unrelated future auto-save. Existing duplicates need one more manual
delete to clear, but they'll stay gone after that.

Folder collapse state now persists (SettingsService.collapsedFolderIds,
JSON-encoded like favoriteModelIds) and is restored on app launch, in
both the sidebar and the advanced conversation list.

Merge model picker: the provider switcher was legitimate (it does load
each provider's own catalog independently) but looked like plain
text — no chevron, no button styling — so it wasn't obviously
clickable. Restyled to match HeaderView's provider menu affordance
(icon + label + chevron on a colored pill).

oAI/Views/Screens/CombineConversationsSheet.swift

@@ -137,11 +137,22 @@ struct CombineConversationsSheet: View {
                                         }
                                     }
                                 } label: {
-                                    Text(mergeProvider.displayName)
+                                    HStack(spacing: 4) {
+                                        Image(systemName: mergeProvider.iconName)
+                                        Text(mergeProvider.displayName)
+                                        Image(systemName: "chevron.up.chevron.down")
+                                            .font(.system(size: 7))
+                                            .opacity(0.7)
+                                    }
+                                    .font(.caption)
+                                    .foregroundColor(.white)
+                                    .padding(.horizontal, 6)
+                                    .padding(.vertical, 3)
+                                    .background(Color.providerColor(mergeProvider))
+                                    .cornerRadius(4)
                                 }
                                 .menuStyle(.borderlessButton)
                                 .fixedSize()
-                                .font(.caption)
                                 .disabled(isProcessing || isLoadingMergeModels)
 
                                 Button("Change Model…") {