Added data backup and restore for chats

The settings gain a data backup panel. It writes the chats of the chosen
workspaces, and optionally the temporary chats, into a single archive file
with the extension .aistudio-chats, and it imports such archives again.
The archive mirrors the chat storage, so restoring needs no conversion.

Documents the user attached are never copied into the archive. The chats
keep the absolute path of each document, which keeps them usable wherever
the documents live and avoids a second copy. Files AI Studio created itself,
such as transcripts, exist nowhere else; when the selection holds any, the
user decides whether they travel with the archive.

On import, chats which already exist, in any workspace, are either kept or
imported as additional chats with a new ID. Workspaces are merged by their
identity. An imported chat gets its own copy of every transcript the archive
did not carry, as a copied chat does, so deleting it never removes the
transcripts of the chat it came from.

An existing archive is only replaced once the new one is complete. Chats
which could not be read are reported instead of missing silently. Both
directions can be stopped at any time and report what was written.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app/MindWork AI Studio/Tools/Rust/FileTypes.cs

@@ -1,3 +1,4 @@
+using AIStudio.Tools.ChatArchive;
 using AIStudio.Tools.PluginSystem;
 
 // ReSharper disable MemberCanBePrivate.Global
@@ -104,6 +105,7 @@ public static class FileTypes
     public static readonly FileTypeFilter EXECUTABLES = FileTypeFilter.Leaf(TB("Executable"), "exe", "app", "bin", "appimage");
     public static readonly FileTypeFilter SHORTCUT = FileTypeFilter.Leaf(TB("Shortcut"), "lnk");
     public static readonly FileTypeFilter PLUGIN_ARCHIVE = FileTypeFilter.Leaf(TB("Plugin archive"), PluginArchive.PLUGIN_FILE_EXTENSION.TrimStart('.'), "zip");
+    public static readonly FileTypeFilter CHAT_ARCHIVE = FileTypeFilter.Leaf(TB("Chat archive"), ChatArchiveFormat.FILE_EXTENSION.TrimStart('.'), "zip");
     
     /// <summary>
     /// The file types AI Studio converts using Pandoc.