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/Services/GitSyncService.swift

@@ -174,9 +174,40 @@ class GitSyncService {
             log.debug("Exported: \(filename)")
         }
 
+        // Remove files for conversations that no longer exist locally (e.g. deleted since
+        // the last export). Without this, a deletion is never reflected in the sync repo,
+        // so importAllConversations() silently resurrects it on every future pull.
+        let currentIds = Set(conversations.map { $0.id.uuidString })
+        let existingFiles = (try? FileManager.default.contentsOfDirectory(atPath: conversationsDir)) ?? []
+        let mdFilesWithContent: [(filename: String, markdown: String)] = existingFiles
+            .filter { $0.hasSuffix(".md") }
+            .compactMap { filename in
+                guard let markdown = try? String(contentsOfFile: conversationsDir + "/" + filename, encoding: .utf8)
+                else { return nil }
+                return (filename, markdown)
+            }
+        for filename in Self.orphanedExportFilenames(currentIds: currentIds, files: mdFilesWithContent) {
+            try? FileManager.default.removeItem(atPath: conversationsDir + "/" + filename)
+            log.info("Removed orphaned export for deleted conversation: \(filename)")
+        }
+
         await updateStatus()
     }
 
+    /// Given the current conversation IDs and the (filename, markdown content) pairs found in
+    /// the sync repo's conversations directory, returns the filenames whose export ID doesn't
+    /// match any current conversation — i.e. files safe to delete because their conversation
+    /// was removed from the database since the last export.
+    nonisolated static func orphanedExportFilenames(
+        currentIds: Set<String>,
+        files: [(filename: String, markdown: String)]
+    ) -> [String] {
+        files.compactMap { file in
+            guard let export = try? ConversationExport.fromMarkdown(file.markdown) else { return nil }
+            return currentIds.contains(export.id) ? nil : file.filename
+        }
+    }
+
     /// Import conversations from markdown files
     func importAllConversations() async throws -> (imported: Int, skipped: Int, errors: Int) {
         try ensureCloned()
@@ -424,6 +455,15 @@ class GitSyncService {
         }
     }
 
+    /// Fire-and-forget sync trigger for conversation-deletion call sites. No-ops when sync
+    /// isn't configured. Deletions otherwise only reach the sync repo on the next incidental
+    /// auto-sync (or never, if the app is closed first) — this makes the removal propagate
+    /// promptly instead of the deleted conversation silently reappearing on next pull+import.
+    func syncAfterDeletion() {
+        guard settings.syncConfigured else { return }
+        Task { await autoSync() }
+    }
+
     /// Perform auto-sync with debouncing (export + push)
     /// Debounces multiple rapid sync requests to avoid spamming git
     func autoSync() async {