Arcanea Desktop — Tauri app with Guardian TUI

## Context
OpenCode already has a Tauri desktop app in `packages/desktop`. Rebrand and enhance with Arcanea identity.

## Tasks
- [ ] Rebrand desktop app: icon, name, splash screen
- [ ] Guardian sidebar with agent status
- [ ] Worktree panel showing parallel sessions
- [ ] Cosmic theme with aurora background
- [ ] System tray with Guardian notifications