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/Models/SyncModels.swift

@@ -68,7 +68,7 @@ struct SyncStatus: Equatable {
     var remoteStatus: String? // "up-to-date", "ahead 3", "behind 2", etc.
 }
 
-struct ConversationExport {
+nonisolated struct ConversationExport {
     let id: String
     let name: String
     let createdAt: Date
@@ -76,7 +76,7 @@ struct ConversationExport {
     let primaryModel: String? // Primary model used in conversation
     let messages: [MessageExport]
 
-    struct MessageExport {
+    nonisolated struct MessageExport {
         let role: String
         let content: String
         let timestamp: Date
@@ -85,7 +85,7 @@ struct ConversationExport {
         let modelId: String? // Model that generated this message
     }
 
-    func toMarkdown() -> String {
+    nonisolated func toMarkdown() -> String {
         var md = "# \(name)\n\n"
         md += "**ID**: `\(id)`\n"
         md += "**Created**: \(ISO8601DateFormatter().string(from: createdAt))\n"
@@ -129,7 +129,7 @@ struct ConversationExport {
     }
 
     /// Parse markdown back to ConversationExport
-    static func fromMarkdown(_ markdown: String) throws -> ConversationExport {
+    nonisolated static func fromMarkdown(_ markdown: String) throws -> ConversationExport {
         let lines = markdown.components(separatedBy: .newlines)
         var lineIndex = 0