Unified UI crate for VT Code: design system, theme registry, and TUI framework.

Module layout

• [design] — Color conversion, style bridging, layout, diff, panel primitives
• [theme] — Theme registry, runtime state, syntax theme resolution
• [tui] — Full TUI framework (session, widgets, runner, markdown, etc.)

Items from design and theme are also re-exported at the crate root for backward-compatibility with callers that previously imported from the standalone vtcode-design / vtcode-theme crates (now consolidated into vtcode-ui).