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/ConversationMergeService.swift

@@ -94,6 +94,7 @@ enum ConversationMergeService {
             for id in conversationIds {
                 _ = try? DatabaseService.shared.deleteConversation(id: id)
             }
+            GitSyncService.shared.syncAfterDeletion()
         }
 
         Log.db.info("Combined \(conversationIds.count) conversations into '\(name)' (mode: \(mode.rawValue), deleteOriginals: \(deleteOriginals))")