[FEATURE]: Rounded corners for TUI code blocks and input box

Describe the enhancement you want to request

Add optional rounded corners for the code-block panels and the input box in the TUI (opencode cli), configurable via theme or tui config.

Why

With a compositor that rounds window corners (e.g. Hyprland decoration.rounding), the rounded terminal frame clashes with the hardcoded square-cornered borders opencode draws around assistant code blocks and the input field. A rounded style would look consistent with rounded terminal windows.

Details

• The TUI renders these borders with hardcoded square box-drawing characters (┌┐└┘); the rounded variants (╭╮╰╯) are not present in the binary at all.
• The theme JSON format only exposes colors (background, border, text, diff/markdown/syntax colors) — there is no border-style or corner-radius option.
• tui.json similarly has no UI style option beyond theme.

Proposal

Support a border style (e.g. "round" / "square") in the theme or tui.json, defaulting to the current square corners so existing setups are unchanged. Since the TUI is built on opentui, if the underlying renderer already supports rounded corners this may just be exposing an existing option.

Environment: opencode 1.18.18 (opentui TUI), Linux/Hyprland.