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/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,