Sync conversation notes via Git Sync, add discard shortcut to crash-recovery prompt

Notes files now export to notes/ + notes.json alongside conversations.json,
matching folders.json's manifest pattern: matched by conversation ID, never
overwrites notes a machine already has locally, same empty-state orphan-
cleanup safety guard as the existing conversation/folder sync code. Also
adds Cmd+D to the "Discard" button on the crash-recovery restore prompt.

Fixes two Swift 6 actor-isolation build warnings surfaced along the way:
ConversationNotesService and the String filename-sanitizing extension are
pure, state-free helpers called from nonisolated contexts (DatabaseService,
GitSyncService) but defaulted to @MainActor — marked nonisolated.

oAI/Models/SyncModels.swift

@@ -86,6 +86,20 @@ nonisolated struct FolderSyncManifest: Codable {
     var assignments: [String: String]
 }
 
+/// Serialized as `notes.json` at the sync repo root, alongside a `notes/` directory holding the
+/// raw note file content (same format as `~/Library/Application Support/oAI/notes/`, see
+/// ConversationNotesService). Matching is by conversationId via this manifest, not by parsing the
+/// embedded `**ID**:` header inside each note file — same approach as `FolderSyncManifest`.
+nonisolated struct NotesSyncManifest: Codable {
+    nonisolated struct Entry: Codable {
+        let filename: String
+        let enabled: Bool
+    }
+
+    /// conversationId -> notes entry. Only present for conversations that have ever had notes.
+    var notes: [String: Entry]
+}
+
 nonisolated struct ConversationExport {
     let id: String
     let name: String