TUI dialogs sit too low once content grows

Description

Tall TUI dialogs are not vertically centered. The host overlay in packages/tui/src/ui/dialog.tsx sets paddingTop to terminal height / 4 and never sets justifyContent="center", so the panel top is pinned at 25% and extra height grows downward.

Short dialogs look roughly centered. After the same dialog grows (plugin panel load, more rows), it sits too low and can clip the bottom. Plugin api.ui.dialog has no position or align field, so callers cannot correct this.

I hit this with plugin panels such as /dcp. Built-in select dialogs show the same top pin when they get tall.

Related but not the same: #37621 (plugin dialogs in the bottom-right), #7902 (overflow, closed as duplicate), and #44754 (replace() resets setSize()).

Plugins

@tarquinen/opencode-dcp is enough to see it via /dcp. Any plugin that calls api.ui.dialog.replace() hits the same host overlay.

OpenCode version

1.18.22

Steps to reproduce

1. Open a session in the TUI.
2. Open a tall plugin dialog (/dcp, or any api.ui.dialog.replace() panel).
3. Compare a short loading or empty state with the full panel.
4. The expanded panel starts about a quarter of the way down instead of staying vertically centered.

Screenshot and/or share link

Operating System

Linux

Terminal

Not terminal-specific. Reproduced from the 1.18.22 Dialog layout: alignItems="center" with paddingTop={dimensions().height / 4} and no justifyContent.