Add terminal color palette option

[rodrigomencos]

Summary

• Add an fx or terminal color palette preference to /settings
• Let terminal themes supply colors through the default foreground, background, and ANSI color slots
• Keep the existing fx palette as the default
• Preserve external command and tool ANSI output

This allows terminal themes to style fx without embedding theme-specific color tables.

Design

• The typed contract is ColorPalette { fx, terminal }
• The preference is profile-owned and stored in ~/.fx/settings.json
• Project .fx.json cannot set the preference
• Palette changes update fx-owned UI and newly generated output immediately and persist across launches
• transcript and external ANSI content remain unchanged.
• Interactive fx and terminal-formatted fx ask use the selected palette

Verification

• zig fmt --check src/
• git diff --check
• zig build
• Focused palette, configuration, persistence, presentation-snapshot, nested-runtime, and transcript-cache tests
• Exercised ./zig-out/bin/fx under Nushell
• Verified live UI and new-output switching in both directions without rewriting retained output
• Verified persistence after restarting fx
• Verified terminal mode emits default and ANSI-16 colors
• Verified external ANSI content remains unchanged