feat(auth): add Supabase passkey support + stabilize backend Flow startup

[zenabot27]

Summary

• add passkey auth support integrated with Supabase via supakeys
• add local passkey auth edge function + DB migration + gateway routing
• wire developer login page with passkey sign-in/register actions
• harden backend startup against Flow endpoint instability (retry + multi-node defaults)

Key Changes

• Frontend
  • passkey client integration in auth context
  • /developer/login passkey UX + support detection
  • Docker build args/env for Supabase/passkey config
• Supabase
  • new function: supabase/functions/passkey-auth/index.ts
  • new migration: supabase/migrations/20260301144541_passkey_auth.sql
  • new gateway config: supabase/gateway/nginx.conf
  • compose services for passkey-auth + supabase-gateway
• Backend reliability
  • Flow client startup changed from fail-fast to retry loop
  • default multi-node FLOW_ACCESS_NODES in compose
  • env example updated for Flow node overrides

Validation

• frontend build: bun run build
• frontend lint: bun run lint (no errors, existing warnings remain)
• function type-check: deno check --node-modules-dir=auto supabase/functions/passkey-auth/index.ts
• backend tests: cd backend && go test ./...
• docker runtime check:
  • backend container stays running with RestartCount=0
  • passkey API smoke tested on /functions/v1/passkey-auth

Notes

• this PR keeps existing behavior for non-passkey login paths
• full WebAuthn hardware flow (register/finish + login/finish) requires manual browser authenticator interaction