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/frigate-audio.sh

@@ -98,8 +98,7 @@ install_frigate-audio() {
 
     # ── Frigate public URL ─────────────────────────────────────────────────────
     echo ""
-    local BASE_DOMAIN=""
-    [ -f "$DOCKER_DIR/.config" ] && BASE_DOMAIN=$(grep '^BASE_DOMAIN=' "$DOCKER_DIR/.config" 2>/dev/null | cut -d= -f2-)
+    local BASE_DOMAIN="${SITE_DOMAIN:-}"
     local FRIGATE_PUBLIC_URL=""
     if [ -n "$BASE_DOMAIN" ]; then
         local _PFX=""