Support for uploads larger than 15 MB

[MasterK0927]

Currently, file uploads are limited to 15 MB due to concerns about blocking the main thread when handling larger files.

Relies on #124

Possible Solution:

• Creating a web worker for handling the upload task, so that it doesn't blocks the main thread.

[MasterK0927]

Initially, we suspected that the main thread was being blocked because of the .readAsDataUrl method used for reading the file, likely due to the overhead of handling large Base64-encoded data. However, even after offloading the file reading process to the Tauri backend by spawning it in a separate thread, the main thread still appears to be blocked.

This leads me to believe the issue might instead be related to how ImageStructs is being handled in ConversationStore.ts.