feat: support configuring default server URL for desktop

[athal7]

Allow the desktop app to connect to a pre-existing server instead of spawning its own sidecar.

Use Case

Users running opencode server separately (e.g., in a container, on a remote machine, or as a shared service) should be able to configure the desktop app to connect to it directly.

Proposed Solution

• Add UI in server selection dialog to set/clear the default server URL
• On startup, attempt to connect to configured server
• Show connection failure dialog with Retry/Start Local options when server is unreachable