fix: static IPs for all containers, radicale config, DNS zone, cleanup

- Assign static IPs to all 13 containers (172.20.0.2–13) so DNS zone
  records match actual container IPs regardless of start order.
- Update cell.zone: all .cell domains now point to cell-caddy (172.20.0.2)
  which is the correct single entry point via Caddy reverse proxy.
- Create config/radicale/config so the calendar container actually starts.
- Fix webdav: replace empty users.passwd with USERNAME/PASSWORD env vars.
- Fix DNS fallback IP in wireguard_manager: 172.20.0.2→172.20.0.3 (cell-dns).
- Remove duplicate http://ui.cell from Caddyfile.
- Add persistent data volumes for rainloop and filegator.
- Fix mail domainname placeholder (yourdomain.com→cell.local).

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

config/radicale/config

@@ -0,0 +1,11 @@
+[server]
+hosts = 0.0.0.0:5232
+
+[auth]
+type = none
+
+[storage]
+filesystem_folder = /data/collections
+
+[logging]
+level = warning