perf: show window immediately during desktop startup

[usvimal]

Summary

Previously the desktop app window was only created after the sidecar server was ready (up to 7 seconds). Users saw nothing during this time. Now the window appears immediately with a loading screen.

Changes

Rust (lib.rs)

• Create window synchronously in setup() instead of inside async spawn
• Add serverReady = false to initialization script
• Dispatch opencode:server-ready event when server is ready

TypeScript (app.tsx)

• Add DesktopServerGate component that shows loading screen
• Listen for opencode:server-ready event to hide loading
• Show animated logo with "Starting server..." text

Before/After

Before	After
Blank screen for 0-7s	Window appears instantly
No feedback	Shows loading animation
Feels broken/slow	Feels responsive

Testing

• Launch desktop app
• Should see loading screen immediately
• Loading screen disappears when server is ready