Split theming into color themes and UI themes

[oobagi]

Summary

Split the current unified Theme struct into two independent layers:

1. Color Theme — palette only (primary, accent, muted, border, status colors, background type)
2. UI Theme — structural/layout choices that change how the program looks and feels

Currently everything lives in one Theme struct in internal/theme/theme.go. This bundles colors with structural decisions (list markers, divider chars, quote bars, checklist glyphs, code label alignment, heading styles). The split would let users mix and match — e.g. "ocean" colors with a "minimal" UI, or "dracula" colors with a "retro" UI.

Color Theme (palette)

What it controls — same as today's core color fields:

• Primary, Success, Error, Warning, Muted, Border, Accent
• StatusBg, StatusFg
• Background ("dark" / "light")

UI Theme (structure & layout)

What it controls — things that change the program's shape, not its colors:

• Footer design — different footer layouts/styles
• Help menu — card overlay vs. bottom popup from footer vs. other layouts
• Background color / fill — whether the app fills the terminal or floats content
• Icons / ASCII art — glyph sets, decorative elements
• Editor/viewer chrome — left sidebar presence, element picker style
• Block styles — list markers, checklist glyphs, divider chars, quote bars, heading decoration, code label alignment (currently BlockStyles)
• General layout — spacing, padding, chrome density

Config

color_theme = "ocean"
ui_theme = "minimal"

Task list

• Define ColorTheme and UITheme structs
• Refactor Theme to compose both (or replace entirely)
• Update FromName / preset loading to resolve color + UI independently
• Add CLI commands (notebook color-theme, notebook ui-theme or extend existing notebook theme)
• Migrate existing presets — each current preset maps to one color theme + one UI theme
• Ship a few distinct UI themes (e.g. default, minimal, retro)
• Update config loading/saving for the two fields

[oobagi]

Closing as not planned.