feat(installer): ask GitHub username first in git screen

screen_git now prompts GitHub username before git name/email, and the
screen description plus installation summary list SSH before git to
match the new prompt order. Behavior unchanged - prompts are
independent and all assignment sites are read after the screen returns.

airootfs/usr/local/bin/alvin-install

@@ -316,15 +316,15 @@ screen_git() {
     echo ""
     gum style --foreground 15 \
         "Optional steps - press Enter on any field to skip it." \
-        "Git config sets user.name/user.email for new repositories;" \
-        "the GitHub username imports that account's SSH public keys."
+        "The GitHub username imports that account's SSH public keys;" \
+        "Git config sets user.name/user.email for new repositories."
     echo ""
+    GH_USERNAME=$(gum input --placeholder "octocat (empty to skip)" \
+        --header "+ GitHub username for SSH keys (optional)") || die "Git configuration cancelled."
     GIT_NAME=$(gum input --value "$USERNAME" --header "+ Git name (Enter keeps '${USERNAME}')") \
         || die "Git configuration cancelled."
     GIT_EMAIL=$(gum input --placeholder "user@example.com (empty to skip)" \
         --header "+ Git email (optional)") || die "Git configuration cancelled."
-    GH_USERNAME=$(gum input --placeholder "octocat (empty to skip)" \
-        --header "+ GitHub username for SSH keys (optional)") || die "Git configuration cancelled."
     log "Git: name='${GIT_NAME}' email='${GIT_EMAIL}' github='${GH_USERNAME}'"
 }
 
@@ -398,8 +398,8 @@ screen_summary() {
         "Locale:      ${LOCALE}" \
         "Timezone:    ${TIMEZONE}" \
         "Login mgr:   ${LOGIN_MANAGER}" \
-        "Git:         ${git_line}" \
-        "SSH keys:    ${ssh_line}"
+        "SSH keys:    ${ssh_line}" \
+        "Git:         ${git_line}"
 
     echo ""
     echo "TPM2 auto-unlock (if applicable) and autologin are asked about during installation."