Vellium v1.0.0 released: security hardening, wallpaper-based themes, JSON chat export and a major desktop stability pass

Vellium has reached v1.0.0. It is a local-first desktop workspace for writing, roleplay, character creation, lorebooks and knowledge management with local LLMs. This release promotes the previous v1.0.0-beta build to the first stable version. The main focus was security and desktop stability. I added stricter Electron security policies, validation for request origins, tighter plugin manifest checks and a global React error boundary. I also reworked parts of the Electron startup and preload flow to fix issue