feat(tui): Sprint 3 — theme system with 11 built-in themes
- Create src/theme.rs with TuiTheme struct (40+ color roles per theme)
- ColorDef enum: Named/Ansi256/Rgb with serde support
- 11 built-in themes: default, tokyonight, catppuccin-mocha, catppuccin-latte,
nord, gruvbox, dracula, solarized-dark, solarized-light, monokai, system
- Theme integrated into TuiApp (tc() helper, set_theme() method)
- Dashboard rendering now uses theme colors (all Color::White/Gray/etc replaced)
- Input area uses theme colors (border, fg, cursor)
- JSON serialization/deserialization for custom themes
- 8 new tests: all builtins, ColorDef variants, JSON round-trip
- 248 unit tests pass
Sprint 3 of 7.
Authored by TheArchitectit
e68a80b418