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

[github-actions]

This issue might be a duplicate of or related to existing issues. Please check:

• [FEATURE]: OpenCode-Desktop Remote Workspace #5608: OpenCode-Desktop Remote Workspace - requests remote development workflow similar to VS Code
• feat(desktop): Add option to run OpenCode backend via WSL on Windows #5635: Add option to run OpenCode backend via WSL on Windows - discusses configuring backend connection options for desktop
• [FEATURE]: Named Remote Connections with Session Origin Tracking #6461: Named Remote Connections with Session Origin Tracking - proposes comprehensive remote connection management with named connections
• Desktop crashes when opening project after attach to remote server #6835: Desktop crashes when opening project after attach to remote server - reports issues with desktop connecting to remote servers

While these issues relate to connecting desktop to remote servers, #7371 specifically focuses on configuring a default server URL for the desktop app itself. Feel free to reference any of these if your use case overlaps!