refactor: restructure installation to modular architecture

- Remove monolithic bootstrap.sh in favor of modular design
- Add install.sh as new main entry point with improved UX
- Split functionality into focused modules:
  - lib/common.sh: shared utilities and environment detection
  - lib/packages.sh: base system packages
  - lib/neovim.sh: Neovim and LazyVim setup
  - lib/arduino.sh: Arduino CLI and board cores
  - lib/git.sh: Git and Lazygit configuration
  - lib/shell-config.sh: Zsh and shell environment
  - lib/starship.sh: Starship prompt setup
  - lib/zellij.sh: Zellij terminal multiplexer
  - lib/scaffold.sh: project scaffolding utilities
- Update README with new installation instructions and architecture
- Improve error handling and user feedback across modules
- Maintain compatibility with both Arch Linux and Ubuntu/WSL2

This modular approach enables better maintainability, testing,
and allows users to selectively run components as needed.

lib/zellij.sh

@@ -0,0 +1,43 @@
+#!/bin/bash
+# Step 4: Zellij terminal multiplexer
+
+step "4/9 — Installing Zellij"
+
+if ! command -v zellij &>/dev/null; then
+  if $IS_ARCH; then
+    $PKG_INSTALL zellij
+  else
+    curl -kL https://github.com/zellij-org/zellij/releases/latest/download/zellij-x86_64-unknown-linux-musl.tar.gz | tar xz -C /tmp
+    sudo mv /tmp/zellij /usr/local/bin/
+  fi
+fi
+info "Zellij installed"
+
+mkdir -p ~/.config/zellij/layouts
+write_zellij_config ~/.config/zellij/config.kdl
+
+cat >~/.config/zellij/layouts/arduino.kdl <<'LAYOUT'
+layout {
+    tab name="code" focus=true {
+        pane split_direction="vertical" {
+            pane size="65%" {
+                command "nvim"
+                args "."
+            }
+            pane split_direction="horizontal" {
+                pane size="60%" name="terminal"
+                pane size="40%" name="serial" {
+                    command "bash"
+                    args "-c" "echo '⚡ Serial — run: tio /dev/ttyUSB0 -b 115200'"
+                }
+            }
+        }
+    }
+    tab name="git" {
+        pane {
+            command "lazygit"
+        }
+    }
+}
+LAYOUT
+info "Zellij configured with $THEME_DISPLAY_NAME + Arduino layout"