Align WA registration flow and proxy lease handling

[pood1e]

Summary

• align WA registration /v2/code and /v2/register native request material with reverse findings
• add optional proxy-runtime registration lease support with WA_REGISTRATION_PROXY_LEASE_MODE
• improve registration channel/no_routes handling, account-transfer flow, OTP extraction expiry, and dashboard channel/error visibility
• add investigation docs and focused registration probe scripts

Validation

• git diff --check main..dev
• remote deploy/scripts/deploy-remote.sh wa-app-service
• Helm lint/template passed during remote deployment
• rollout verified for byte-v-forge-wa-app-service
• deployed image: byte-v-forge/wa-app-service:deploy-20260618221114
• Helm revision: 1455