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/starship.sh

@@ -0,0 +1,17 @@
+#!/bin/bash
+# Step 3: Starship prompt
+
+step "3/9 — Installing Starship"
+
+if ! command -v starship &>/dev/null; then
+  if $IS_ARCH; then
+    $PKG_INSTALL starship
+  else
+    curl -ksS https://starship.rs/install.sh | sh -s -- -y
+  fi
+fi
+info "Starship installed"
+
+mkdir -p ~/.config
+write_starship_config ~/.config/starship.toml
+info "Starship configured with $THEME_DISPLAY_NAME"