feat(themes): add theme system with starship integration

- Add theme selection system with 5 predefined themes
- Implement Catppuccin Mocha, Dracula, Gruvbox Dark, Nord, and Tokyo Night
- Each theme includes NvChad integration and starship prompt configuration
- Support for custom color palettes and consistent formatting
- Interactive theme selection with numbered menu

lib/themes.sh

@@ -0,0 +1,30 @@
+#!/bin/bash
+# Theme selection — sources the chosen theme file
+
+THEMES_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)/themes"
+
+THEME_FILES=(gruvbox catppuccin tokyonight nord dracula)
+
+select_theme() {
+  echo "  Select colorscheme:"
+  echo ""
+
+  local i=1
+  for t in "${THEME_FILES[@]}"; do
+    source "$THEMES_DIR/$t.sh"
+    printf "    %d) %s\n" "$i" "$THEME_DISPLAY_NAME"
+    i=$((i + 1))
+  done
+
+  echo ""
+  read -rp "  Choice [1-${#THEME_FILES[@]}]: " theme_choice
+
+  # Default to 1 (Gruvbox) for invalid input
+  if [[ ! "$theme_choice" =~ ^[0-9]+$ ]] || ((theme_choice < 1 || theme_choice > ${#THEME_FILES[@]})); then
+    theme_choice=1
+  fi
+
+  local selected="${THEME_FILES[$((theme_choice - 1))]}"
+  source "$THEMES_DIR/$selected.sh"
+  info "Selected theme: $THEME_DISPLAY_NAME"
+}