fix(tui): show warning when opening WebUI without server mode

[lxxonx]

Summary

Fixes #8848

When OpenCode runs without --port flag, the "Open WebUI" command attempts to open http://opencode.internal which is a non-resolvable internal URL used only for RPC communication.

Changes

• Added a check in the "Open WebUI" command handler to detect when the URL is the internal placeholder
• Shows a helpful warning toast explaining how to enable server mode instead of silently failing

Before

Clicking "Open WebUI" → Browser opens with http://opencode.internal → ERR_NAME_NOT_RESOLVED

After

Clicking "Open WebUI" → Toast warning: "WebUI requires server mode. Restart with: opencode --port 4096"

Testing

1. Run opencode (without --port flag)
2. Press Cmd+P → Select "Open WebUI"
3. Verify warning toast appears instead of browser opening

Alternative Approaches Considered

1. Hide the menu item when server not running - Would require passing server state through context, more invasive change
2. Start server on-demand when WebUI requested - Adds complexity and may have side effects
3. Show warning (chosen) - Minimal change, clear user feedback, explains the fix

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Found potential duplicates:

Related PRs:

1. PR fix(tui): start server on-demand when opening WebUI from command palette #8409 - fix(tui): start server on-demand when opening WebUI from command palette

   • fix(tui): start server on-demand when opening WebUI from command palette #8409
   • This addresses the same underlying issue (WebUI not working without server mode) but proposes an alternative approach: starting the server on-demand instead of just showing a warning.

2. PR feat(cli): rework server for webui (safe by default) #8546 - feat(cli): rework server for webui (safe by default)

   • feat(cli): rework server for webui (safe by default) #8546
   • This appears to be a broader rework of the server/WebUI functionality that may be related to the same issue space.

These PRs likely handle the same problem domain. PR #8409 is the closest match since it directly addresses the "Open WebUI" behavior when the server isn't running, though with a different solution (auto-start vs. warning).