feat: add networkd subsystem and fix code review issues

Phase 1-4: Networkd subsystem
- lib/network.py: systemd-networkd config renderer (.network INI files)
  with full schema support: [Match], [Link], [Network], [Address], [Route],
  [DHCPv4], [DHCPv6] sections. One Address/=DNS= line per value per spec.
  Route sections use #N suffix per systemd.syntax(7).
- lib/network.py: generate_network_files() with 50-<name>.network prefix
  and stale file cleanup
- lib/network.py: collect_upstream_dns() filters local/private DNS
- lib/network.py: infer_dhcp_ranges() and infer_zones() helpers
- daemon/handlers/network.py: routes for GET/POST /network/interfaces
  and full apply with DNS upstream sync to dnsmasq
- webui/api/network.py: Flask blueprint for /api/network/* endpoints
- webui/api: interfaces page updated with IP config inline editing
- lib/state.py: networkd collector using parse_networkctl_status()
- system/sudoers.d/vacuum-walld: networkctl + systemd-network rules
- system/systemd/vacuum-walld.service: ReadWritePaths for /etc/systemd/network
- install.sh: ACME email now optional, configured from WebUI
- lib/acme.py: get_email() falls back to declarative config

Phase 5: Code review fixes
- daemon/server.py: path params now win over JSON body and query params
  in request body merge (prevents config save name override)
- daemon/server.py: remove dead 'import re'
- daemon/handlers/network.py: replace Path.mkdir() with sudo mkdir
  for /etc/systemd/network (ProtectSystem=strict compatibility)
- system/sudoers.d/vacuum-walld: pin systemctl to specific commands
  (reload/is-active dnsmasq instead of wildcard)
- system/sudoers.d/vacuum-walld: restore !requiretty and section comment
- lib/network.py: remove unused _MANAGEMENT_PORTS constant
- webui/api/network.py: remove redundant body[\name\] = name in save_interface

Tests: 332 passing (110 new/updated), ruff clean

tests/test_firewall.py

@@ -314,7 +314,6 @@ _FakeState = {
         "interfaces": [
             {
                 "name": "eth0",
-                "display_name": "eth0",
                 "mac": "aa:bb:cc:dd:ee:00",
                 "state": "UP",
                 "mtu": 1500,
@@ -324,7 +323,6 @@ _FakeState = {
             },
             {
                 "name": "eth1",
-                "display_name": "eth1",
                 "mac": "aa:bb:cc:dd:ee:01",
                 "state": "UP",
                 "mtu": 1500,