feat(amneziawg): Phase 2a — IPv6 support + NDP proxy

Adds native dual-stack IPv6 to AmneziaWG inbounds, ported from
coinman-dev/3ax-ui's approach:

- ServerSettings gets ipv6Enabled/ipv6Subnet/ipv6ExternalInterface;
  Instance carries the server's own IPv6 address (first host of the
  subnet) alongside its IPv4 one.
- defaultAmneziaWGClients allocates an IPv6 host address per client
  (second AllowedIPs entry) when the server has IPv6 enabled, reusing
  allocateWireguardAddress — which needed a real fix along the way: it
  always suffixed "/32" regardless of address family, which is wrong
  for an IPv6 host address (needs /128). Now family-aware.
- generateServerConfig's PostUp/PostDown gains IPv6 forward-accept
  rules, proxy_ndp sysctl, and one `ip -6 neigh add/del proxy` entry per
  enabled peer with an IPv6 address — the lightweight per-client
  method, not the ndppd-daemon whole-subnet method (not worth the
  config-file-management complexity at this scale; ndppd itself is
  still installed by install.sh in case that changes later).
- ValidateIPv6Subnet rejects a malformed subnet before save.
- Frontend: ipv6Enabled/ipv6Subnet/ipv6ExternalInterface fields on the
  AmneziaWG inbound form, EN+RU translations, openapi.json/generated/*
  regenerated (the latter via `go run ./tools/openapigen`, pure Go).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/public/openapi.json

@@ -2821,6 +2821,15 @@
           "i1": {
             "type": "string"
           },
+          "ipv6Enabled": {
+            "type": "boolean"
+          },
+          "ipv6ExternalInterface": {
+            "type": "string"
+          },
+          "ipv6Subnet": {
+            "type": "string"
+          },
           "jc": {
             "description": "Obfuscation20's fields, repeated flat (not embedded) rather than\nnested under their own key: encoding/json would happily inline an\nembedded Obfuscation20 the same way, but the frontend's Go->Zod/TS\ngenerator (tools/openapigen) does not — it emits a genuinely nested\n`obfuscation20` object, which would silently diverge from the real\nwire JSON. See Obfuscation() below for the manager-facing conversion.",
             "type": "integer"