Move thumbnail storage from DB text blobs to file-based cache

[clFaster]

Goal

Move thumbnail storage from DB text blobs to file-based cache

Type

Technical

Effort

M

Impact

Medium

Evidence

Thumbnail is currently stored as TEXT in DB schema (thumbnail TEXT) in src-tauri/src/db.rs and written often from src/components/ExcalidrawFrame.tsx

First Implementation Step

Store thumbnails in app-data files and keep only relative path/hash in DB