feat(theme): add community themes and streamline theme selector

[Ansub]

Summary

• add built-in community themes (dracula, catppuccin-mocha, tokyonight, gruvbox, nord, plus aurora and sage)
• make community themes first-class in discovery/install/doctor paths
• simplify boo theme select to a flat list with tighter spacing, active-row highlight, and right-side terminal preview
• order selector/list as built-in dark -> built-in light -> community dark -> community light, with a visual gap before community
• remove custom selector filter keybinds and keep simple Enter/Esc flow
• update CI theme validation to include nested themes/community/*.theme
• update README for the new selector behavior

Validation

• bash -n bin/boo
• bash -n scripts/install.sh
• ./bin/boo theme list
• temp-HOME smoke for ./bin/boo theme select and ./bin/boo theme <name> applies