fix(amneziawg): fill in IPv6 fields missed by the Phase 2a commit

Two real gaps the CI caught (both new fields, both my miss):

- inbound-defaults.ts's createDefaultAmneziawgInboundSettings() built a
  server object literal predating ipv6Enabled/ipv6Subnet/
  ipv6ExternalInterface — AmneziawgServer's inferred type now requires
  them (zod .default() fields are non-optional post-parse), so this
  didn't typecheck at all.
- openapi.json's ipv6Enabled property was missing the description the
  real generator attaches (the Go doc comment covering all three IPv6
  fields is attached to the first one) — a one-line diff, but git
  diff --exit-code doesn't care how small.

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

frontend/public/openapi.json

@@ -2822,6 +2822,7 @@
             "type": "string"
           },
           "ipv6Enabled": {
+            "description": "IPv6Enabled turns on native IPv6 for clients: an IPv6 host address is\nallocated from IPv6Subnet alongside each client's IPv4 one, and the\nserver proxies NDP for each enabled client's address so upstream\nrouters see it as directly reachable (no NAT66). IPv6ExternalInterface\noverrides ExternalInterface for the NDP-proxy PostUp/PostDown entries\nspecifically; empty reuses ExternalInterface.",
             "type": "boolean"
           },
           "ipv6ExternalInterface": {

frontend/src/lib/xray/inbound-defaults.ts

@@ -295,6 +295,9 @@ export function createDefaultAmneziawgInboundSettings(): AmneziawgInboundSetting
       primaryDns: '8.8.8.8',
       secondaryDns: '8.8.4.4',
       externalInterface: '',
+      ipv6Enabled: false,
+      ipv6Subnet: '',
+      ipv6ExternalInterface: '',
       jc: 5,
       jmin: 10,
       jmax: 50,