Implementation: Jan uses cortex.cpp to persist /threads, /messages

[louis-menlo]

Problem Statement

This is the frontend implementation issue for issue #3904, which is related to QA tracking.

This epic scope threads and messages migration to use cortex.cpp APIs instead of persisting via Jan FS APIs

Deprecate file system requests and ensure that all operations are served by cortex.cpp. The scope of this change is to affect the following operations:

• Thread creation
• Thread title update
• Message send and receive (including text, images, and PDFs)
• Thread settings, model settings, and tool settings
• Files upload
• Image upload

Out of scope:

• File ingestion (VectorDB & Retrieval)

Notes:

• All of the current threads from older versions should be maintained
• New file upload now persisted in /files
• Base64 image now also OpenAI compatible data -> attached in messages.jsonl -> No external file preview supported -> only inline preview.