feat: add post-install theme switcher (archduino-theme)

Add standalone theme switcher command that lets users change the global
colorscheme after installation. Updates Starship, Zellij, Lazygit, and
NvChad in one step. Includes `theme` shell alias and README docs.

lib/scaffold.sh

@@ -135,4 +135,113 @@ void loop() {
   delay(1000);
 }
 INO
-info "Scaffolding tool + templates created"
+# ── Install theme files + theme switcher ─────────────────────────
+mkdir -p ~/.config/archduino/themes
+cp "$SCRIPT_DIR/lib/themes/"*.sh ~/.config/archduino/themes/
+
+cat >~/.local/bin/archduino-theme <<'SWITCHER'
+#!/bin/bash
+set -euo pipefail
+
+GREEN='\033[0;32m'
+YELLOW='\033[1;33m'
+RED='\033[0;31m'
+BOLD='\033[1m'
+NC='\033[0m'
+
+info() { echo -e "${GREEN}[✓]${NC} $1"; }
+warn() { echo -e "${YELLOW}[!]${NC} $1"; }
+error() { echo -e "${RED}[✗]${NC} $1"; exit 1; }
+
+THEMES_DIR="$HOME/.config/archduino/themes"
+THEME_SAVE="$HOME/.config/archduino/theme"
+
+[[ ! -d "$THEMES_DIR" ]] && error "Theme files not found. Re-run the Archduino installer."
+
+THEME_FILES=(gruvbox catppuccin tokyonight nord dracula)
+
+# Load saved theme
+saved_theme="" current_idx=0
+if [[ -f "$THEME_SAVE" ]]; then
+  saved_theme=$(cat "$THEME_SAVE")
+  for idx in "${!THEME_FILES[@]}"; do
+    [[ "${THEME_FILES[$idx]}" == "$saved_theme" ]] && current_idx=$((idx + 1)) && break
+  done
+fi
+
+echo ""
+echo -e "  ${BOLD}Archduino Theme Switcher${NC}"
+echo ""
+
+i=1
+for t in "${THEME_FILES[@]}"; do
+  source "$THEMES_DIR/$t.sh"
+  if [[ $i -eq $current_idx ]]; then
+    printf "    %d) %s ${GREEN}(current)${NC}\n" "$i" "$THEME_DISPLAY_NAME"
+  else
+    printf "    %d) %s\n" "$i" "$THEME_DISPLAY_NAME"
+  fi
+  i=$((i + 1))
+done
+
+echo ""
+read -rp "  Choice [1-${#THEME_FILES[@]}]: " choice
+
+if [[ -z "$choice" || ! "$choice" =~ ^[0-9]+$ ]] || ((choice < 1 || choice > ${#THEME_FILES[@]})); then
+  echo "  No change."
+  exit 0
+fi
+
+if [[ $choice -eq $current_idx ]]; then
+  echo "  Already using that theme."
+  exit 0
+fi
+
+selected="${THEME_FILES[$((choice - 1))]}"
+source "$THEMES_DIR/$selected.sh"
+
+echo ""
+info "Applying $THEME_DISPLAY_NAME..."
+
+# Starship
+write_starship_config ~/.config/starship.toml
+info "Starship updated"
+
+# Zellij
+write_zellij_config ~/.config/zellij/config.kdl
+info "Zellij updated"
+
+# Lazygit
+write_lazygit_config ~/.config/lazygit/config.yml
+info "Lazygit updated"
+
+# NvChad
+NVIM_CFG="$HOME/.config/nvim"
+if [[ -f "$NVIM_CFG/lua/chadrc.lua" ]]; then
+  cat >"$NVIM_CFG/lua/chadrc.lua" <<EOF
+---@type ChadrcConfig
+local M = {}
+
+M.base46 = {
+  theme = "$NVCHAD_THEME",
+}
+
+return M
+EOF
+  # Clear theme cache
+  rm -rf ~/.local/share/nvim/lazy/base46/lua/base46/themes 2>/dev/null || true
+  rm -rf ~/.cache/nvim/base46 2>/dev/null || true
+  info "NvChad updated (restart nvim to see changes)"
+fi
+
+# Save choice
+echo "$selected" > "$THEME_SAVE"
+
+echo ""
+info "Theme switched to $THEME_DISPLAY_NAME"
+warn "Restart Zellij and Neovim for full effect."
+echo ""
+SWITCHER
+chmod +x ~/.local/bin/archduino-theme
+
+info "Scaffolding tool, templates, and theme switcher created"