fix(app): different servers with same project path share sessions

[ysm-dev]

When two servers point to the same project directory (e.g. 127.0.0.1:4096 -> ~/git/opencode and 100.88.20.152:4096 -> ~/git/opencode), the desktop/web app shows the same sessions for both.

The cause is that ServerKey in packages/app/src/app.tsx was changed from a reactive <Show when={...} keyed> to a plain if (!server.url) return null. This means switching servers no longer remounts GlobalSDKProvider / GlobalSyncProvider, so their directory-keyed in-memory state (sessions, sync data) is reused across different servers that happen to share the same path.

[github-actions]

This issue might be related to #13211 ('[Regression] Desktop app: SSE event stream broken with custom remote server'), as both are regressions in the desktop app's handling of remote/custom servers. If they stem from the same architectural change in the ServerKey component, fixing one may help with the other.