[Pepps233]

Summary

Unify theme-sensitive UI styling across startup content, selectors, prompt input, tool output, and built-in themes.

Reasoning

Theme previews updated the startup logo border but left other intro content styled with the previous theme. Selection rendering also mixed semantic colors and arrow glyphs, while prompt, tool, and warning colors had inconsistent contrast in related UI states.

This branch makes those states use shared semantic styles and ensures theme-dependent content is recomputed when the active theme changes.

Changes

• Recompute styled startup details during rendering so theme previews and hot reloads update the complete intro.
• Add selectionAccent as the semantic selected-row foreground, with compatibility fallbacks for themes that only define autocompleteSelected.
• Share the > selected prefix and blank unselected prefix across TUI selectors.
• Keep the prompt cursor visible on empty input, align wrapped text after the prompt marker, and preserve editable bash-mode behavior.
• Improve built-in warning, success, accent, tool-state, and dark-theme background colors.
• Render read ranges with the accent color and plan markers with the neutral tool-output color.
• Make the update_plan schema compatible with DeepSeek while preserving its update-or-clear contract.
• Update theme documentation, schema, and regression coverage.

Testing

• npm run check
• node --test --test-reporter=spec test/select-list.test.ts test/settings-list.test.ts from packages/tui
• Focused Vitest coverage for themes, intro rendering, selectors, editor input, and scoped-model regressions
• Interactive ./diffex-test.sh verification in tmux for dark/light theme previews, intro rendering, selection styles, and prompt behavior