[Feature Request]: Desktop app needs GUI for managing plugins and LSP servers, plus visible load-failure errors

Summary

The OpenCode Desktop app is great, but installing/configuring plugins and LSP servers currently requires hand-editing opencode.jsonc (e.g. %USERPROFILE%\.config\opencode\opencode.jsonc). For desktop users this is a major pain point:

• Plugins: you must know the exact package spec (e.g. superpowers@git+https://github.com/obra/superpowers.git vs the plain npm name oh-my-openagent, where a wrong guess pulls an unrelated placeholder package from npm, e.g. the superpowers@0.0.2 squat). One wrong string = silent load failure.
• LSP servers: need the exact binary path + --stdio args + file-extension mapping, hand-written per language.
• There is no error surfacing either — a bad plugin/LSP entry causes silent failures with no dialog in the Desktop UI (users only find out via log files).

Note: the MCP management UI part of the original request is intentionally split out and tracked in #40335 (form-based add/edit of local/remote MCP + "Test Connection"). This issue covers only plugins, LSP, and error visibility.

Proposal

Add a management UI in the Desktop app, ideally under a "Settings" or "Extensions" section:

1. Plugins tab — list installed plugins, add via a documented format, toggle enable/disable, show load status with visible errors (not silent).
2. LSP tab — form-based: language, command, extensions; default presets for common languages.
3. Error surfacing — surface plugin/LSP load failures in the UI (list + details, timestamped) instead of only log files, with links to the relevant log lines. This overlaps the centralized diagnostics idea in #41194 / #34933 / #37223.

Environment

• OpenCode Desktop v1.18.15, Windows 11
• Real-world pain point: today I installed Superpowers — the plugin entry cannot simply be "plugin": ["superpowers"] (npm superpowers@0.0.2 is an unrelated placeholder) — users must understand the full git spec. A GUI with a validated add-flow eliminates this entire class of mistakes.

Related

• #40335: MCP server setup/Test Connection UI (MCP-only, tracked separately)