tui: /models dialog is fixed-size and cannot be maximized #50527

Summary

The TUI /models picker (DialogModel in packages/tui/src/component/dialog-model.tsx) is a fixed-size dialog. With several providers configured the list is cramped, and there is no way to give it more of the terminal.

Split out of #50513, which now tracks the CLI half only.

Current behavior

1. Fixed dialog size — the dialog width comes from a size prop (medium / large / xlarge → 60 / 88 / 116 columns) and does not adapt to the terminal.
2. No maximize — no keyboard shortcut or action-bar button expands the list to fill the terminal.
3. Cramped scroll area — with 80+ models the visible slice is a small fraction of the list.

Expected behavior

• A way to expand the /models dialog to (close to) full terminal width and height.
• Ideally a general Dialog capability rather than one special-cased in DialogModel, so other dialogs can opt in.

Notes

packages/tui/src/ui/dialog.tsx already reads useTerminalDimensions() for paddingTop, so the dimensions are available where the width is computed.

CONTRIBUTING says UI changes need a design review with the core team, so filing this as an issue first rather than opening a PR — happy to implement whatever shape the team prefers (keybinding vs. action-bar toggle vs. size-cycling).