[FEATURE]: TUI plugins (statusline/slots) don't work in Desktop app - add TUI plugin support to Desktop #41441

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Describe the enhancement you want to request

The Desktop app (@opencode-aidesktop) does not load TUI plugins configured in tui.json (e.g. "plugin": ["file:///path/to/plugin"]). TUI plugin slots like session_prompt — used by statusline plugins such as opencode-quota-statusline / opencode-subagent-statusline — only render in the terminal TUI. In the Desktop app there is no equivalent extension point for custom UI widgets, so those plugins silently do nothing.

Reproduced on the same machine with the same config:

• TUI (opencode in Windows Terminal): statusline renders fine above the prompt.
• Desktop app (1.18.15, Windows x64): nothing renders. opencode.log shows loading tui config only for TUI/CLI processes, never for the Desktop process.

Describe the solution you'd like

Add TUI plugin (slot) support to the Desktop / web UI so plugins registered via tui.json (or opencode.json) render in the Desktop app:

1. Load and evaluate TUI plugin modules in the Desktop renderer/web UI (or a shared runtime), including exports["./tui"] entrypoints and api.slots.register().
2. Map the existing host slots (session_prompt, home_prompt, app_bottom, etc.) to the corresponding areas of the web UI.
3. Fallback: if full slot parity is too large, expose at least the session_prompt slot (the statusline use case) in the Desktop UI.

This would unblock statusline plugins (provider quota/usage, sub-agent state) and similar UI plugins for Desktop users, who are mouse/GUI-first by definition.

Additional context

• Complementary to [FEATURE]: Custom statusLine via shell command (like Claude Code) #37464 (custom statusLine via shell command): that one proposes a config-only statusline; this one asks for TUI plugin/slot parity in Desktop.
• Same class of problem as [FEATURE]: Desktop version side session support (like /btw in TUI) #33469 (side sessions in Desktop) — TUI-only plugins that don't work in Desktop.
• Related: feat(opencode): add unified marketplace #40108 (unified marketplace with a shared runtime for Desktop, Web, TUI, CLI) would be a natural place to also unify the plugin/slot runtime.
• Steps to reproduce: put a file:// TUI plugin in ~/.config/opencode/tui.json, run the same opencode install as Desktop and as TUI; the statusline appears only in the TUI.
• Environment: OpenCode Desktop 1.18.15 (Electron, Windows 11); plugin opencode-quota-statusline (local file plugin registered in ~/.config/opencode/tui.json).