Plugin-registered api.ui.Dialog windows are not centered (positioned in bottom-right corner) #37621

Problem

Plugin-registered TUI dialogs via api.ui.Dialog() are positioned in the bottom-right corner of the terminal instead of being centered. Built-in dialogs (e.g. /models → DialogModel) are correctly centered.

Environment

• opencode: 1.18.3
• OS: Linux
• Plugin: oh-my-opencode-slim 2.2.3

Reproduction

1. Install a plugin that registers a TUI slash command with a dialog:

api.command.register(() => [{
  title: "Test",
  value: "test",
  slash: { name: "test" },
  onSelect: () => {
    api.ui.dialog.replace(() =>
      api.ui.Dialog({
        size: "large",
        onClose: () => api.ui.dialog.clear(),
        children: api.ui.DialogSelect({
          title: "Test dialog",
          options: [{ title: "A", value: "a" }],
          onSelect: () => {},
        }),
      }),
    );
  },
}]);

2. Invoke the command from autocomplete
3. Observe: the dialog appears in the bottom-right corner, not centered

Expected behavior

Plugin dialogs should be centered the same way built-in dialogs are.

Analysis

TuiDialogProps only exposes:

type TuiDialogProps = {
    size?: "medium" | "large" | "xlarge";
    onClose: () => void;
    children?: JSX.Element;
};

There is no position, align, or center field. The plugin cannot control dialog placement. Built-in dialogs like DialogModel (packages/tui/src/component/dialog-model.tsx) presumably handle their own positioning internally, but this logic is not available to plugins through the api.ui.Dialog wrapper.

Request

Either:

1. Make api.ui.Dialog center by default (matching built-in dialog behavior), or
2. Add a position / align field to TuiDialogProps so plugins can control placement.

Context

Discovered while developing the oh-my-opencode-slim /preset TUI command. The three-level preset manager works correctly functionally, but all dialogs appear in the bottom-right corner instead of centered. The built-in /models dialog opens correctly centered.