Add site-wide defaults: timezone, domain, Caddy network

Introduces a one-time configuration wizard (sudo ./setup.sh configure)
that stores SITE_TZ, SITE_DOMAIN, and SITE_CADDY_NET in ~/docker/.config.
Every service now uses these as prompt defaults so the user types common
values once instead of re-answering the same questions for each service.

- lib/common.sh: load_site_config / save_site_config; auto-loads on source;
  backward-compat BASE_DOMAIN alias kept for old .config files
- setup.sh: run_site_configure wizard; first-run offer after base install;
  `sudo ./setup.sh configure` command to update defaults at any time
- 14 services: TZ_VAL now honours SITE_TZ, falling back to /etc/timezone
- 3 inline-heredoc services (filebrowser, homeassistant, ntfy): same fix
- authelia: SITE_TZ/SITE_DOMAIN as prompt defaults; SITE_CADDY_NET replaces
  hardcoded caddy_net throughout (env, compose patch, network creation)
- minecraft, frigate-audio: simplify BASE_DOMAIN read to use SITE_DOMAIN
- sky-cam: SITE_TZ as default for timezone prompt

https://claude.ai/code/session_01Y4dMKtkqkpvmgDKoRdzhTG

services/magicmirror.sh

@@ -33,7 +33,7 @@ install_magicmirror() {
     mkdir -p "$MM_BASE"
     chown "$ACTUAL_USER:$ACTUAL_USER" "$MM_BASE"
 
-    local TZ_VAL; TZ_VAL=$(cat /etc/timezone 2>/dev/null || echo "UTC")
+    local TZ_VAL; TZ_VAL="${SITE_TZ:-$(cat /etc/timezone 2>/dev/null || echo UTC)}"
     local i MM_PORT MM_DIR
 
     for i in $(seq 1 "$MM_COUNT"); do