feat: sync billing header cc_version with User-Agent and add opt-in CCH signing

- Sync cc_version in x-anthropic-billing-header with the fingerprint
  User-Agent version, preserving the message-derived suffix
- Implement xxHash64-based CCH signing to replace the cch=00000
  placeholder with a computed hash
- Add admin toggle (enable_cch_signing) under gateway forwarding settings,
  disabled by default

frontend/src/i18n/locales/en.ts

@@ -4268,6 +4268,8 @@ export default {
         fingerprintUnificationHint: 'Unify X-Stainless-* headers across users sharing the same OAuth account. Disabling passes through each client\'s original headers.',
         metadataPassthrough: 'Metadata Passthrough',
         metadataPassthroughHint: 'Pass through client\'s original metadata.user_id without rewriting. May improve upstream cache hit rates.',
+        cchSigning: 'CCH Signing',
+        cchSigningHint: 'Sign the billing header in forwarded requests with CCH hash. When disabled, the placeholder is preserved.',
       },
       site: {
         title: 'Site Settings',