Title: Terminal/TUI Mockup Support (Character Grid + ANSI Colors)
Description:
Add canvas mode for fixed-width terminal UIs:
- Monospace grid (80x24 chars default)
- Per-cell foreground/background (ANSI 256/true color)
- Bold/underline/dim effects
- Import screenshots → OCR grid
- Export: ANSI text + JSON (Ratatui/Crossterm)
Use case: TUI app prototyping (Ratatui dashboards)
