Stage 6: server image with a console installer (0.4.0)

- server/ profile and build-iso.sh [desktop|server]; make iso-server,
  make qemu-server, run-qemu.sh --serial; login on tty and ttyS0
- das-server: SSH, chrony, NetworkManager/dhcpcd, Docker with Compose,
  nftables (closed except SSH, own table), fail2ban reading socklog,
  sshd without root passwords, Docker log rotation, tools
- das-server-install: dialog installer in Ukrainian and English, or
  unattended with --config; GPT + Btrfs subvolumes, GRUB for UEFI and BIOS,
  serial console, SSH key with optional keys-only logins
- das-install-common: mount-source and postinstall shared with Calamares
- dsv: runit service manager (list, enable, disable, start, stop,
  restart, status, logs) with bash completion, also on the desktop
- das-update split into a subpackage without GTK; the window and tray
  icon move to /usr/libexec/das/das-update-gui
- das-snapper-setup enables dbus, which snapper needs
- TODO: ISO size review with measured numbers and options

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

srcpkgs/das-server/files/nftables.conf

@@ -0,0 +1,31 @@
+#!/usr/sbin/nft -f
+# Das OS server firewall: incoming connections are closed except SSH.
+#
+# Only the "das" table is replaced, so loading this file again
+# (dsv restart nftables) keeps the rules of Docker and fail2ban.
+# Ports that Docker containers publish (-p 8080:80) pass through Docker's
+# own forwarding rules, not the input chain below.
+
+table inet das
+delete table inet das
+
+table inet das {
+	chain input {
+		type filter hook input priority filter; policy drop;
+
+		ct state established,related accept
+		ct state invalid drop
+		iif lo accept
+		meta l4proto { icmp, ipv6-icmp } accept
+		# DHCPv6 answers come from the router's link-local address
+		ip6 saddr fe80::/10 udp dport dhcpv6-client accept
+		# Containers may reach services on the host (DNS, a database...)
+		iifname "docker0" accept
+		iifname "br-*" accept
+
+		tcp dport ssh accept
+
+		# Open more ports here, then: dsv restart nftables
+		# tcp dport { http, https } accept
+	}
+}