Add landing page, Hetzner labels, and Codeberg link

- Add default landing page (roles/caddy/templates/index.html.j2) deployed
  to empty caddy sites; adapted from YC/coming-soon by Steven Tang (MIT),
  with site domain and powered-by footer linking to codeberg.org/opennomad/linderhof
- Apply hcloud_labels to all Hetzner cloud and DNS resources; default to {}
  in role defaults for stacks without the variable defined
- Fix setup.sh: export stack_name so envsubst substitutes it in config.yml
- Add Codeberg repo link to README

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

setup.sh

@@ -45,6 +45,8 @@ ok "collections installed"
 echo
 info "stack setup"
 prompt stack_name "Stack name" "home"
+export stack_name
+
 
 LINDERHOF_CONFIG_DIR="${XDG_CONFIG_HOME:-$HOME/.config}/linderhof"
 STACK_DIR="$LINDERHOF_CONFIG_DIR/$stack_name"
@@ -174,7 +176,7 @@ if [[ -f "$CONFIG" ]]; then
   warn "config.yml already exists — skipping (not overwriting)"
 else
   info "writing config.yml..."
-  envsubst '$admin_user $server_name $server_ip $domain $ssh_key_pub' \
+  envsubst '$admin_user $server_name $server_ip $domain $ssh_key_pub $stack_name' \
     < "$TEMPLATES/config.yml.setup" > "$CONFIG"
   ok "config.yml created"
 fi