feat: add Nerd Font installation, Yazi file manager, and themed Yazi config

- Install JetBrainsMono Nerd Font for icons across all TUI tools
- Add Yazi as terminal file manager with themed config per colorscheme
- Add write_yazi_config() to all 5 theme files
- Theme switcher (archduino-theme) now also updates Yazi
- Add 'y' alias for yazi in shell config
- Update step numbering (9 -> 11 steps) and README

lib/zellij.sh

@@ -1,7 +1,7 @@
 #!/bin/bash
 # Step 4: Zellij terminal multiplexer
 
-step "4/9 — Installing Zellij"
+step "5/11 — Installing Zellij"
 
 if $IS_ARCH; then
   $PKG_INSTALL zellij