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/yazi.sh

@@ -0,0 +1,22 @@
+#!/bin/bash
+# Step 8: Yazi file manager
+
+step "8/11 — Installing Yazi"
+
+if $IS_ARCH; then
+  $PKG_INSTALL yazi ffmpegthumbnailer p7zip poppler
+else
+  YAZI_VERSION=$(curl -ks "https://api.github.com/repos/sxyazi/yazi/releases/latest" | jq -r '.tag_name')
+  curl -ksSLo /tmp/yazi.zip "https://github.com/sxyazi/yazi/releases/download/${YAZI_VERSION}/yazi-x86_64-unknown-linux-musl.zip"
+  unzip -qo /tmp/yazi.zip -d /tmp/yazi-extract
+  sudo install -m 755 /tmp/yazi-extract/yazi-x86_64-unknown-linux-musl/yazi /usr/local/bin/yazi
+  rm -rf /tmp/yazi.zip /tmp/yazi-extract
+  # Optional thumbnailer/preview deps
+  sudo apt install -y ffmpegthumbnailer p7zip-full poppler-utils 2>/dev/null || true
+fi
+info "Yazi installed/updated"
+
+mkdir -p ~/.config/yazi
+backup_config ~/.config/yazi/theme.toml
+write_yazi_config ~/.config/yazi/theme.toml
+info "Yazi configured with $THEME_DISPLAY_NAME"