fix(conversations): load with internal user id

[rissrice2105-agent]

Summary

• resolve the authenticated Supabase user to the internal users.id before loading conversations
• query conversation_participants with the internal user id instead of auth_user_id
• add regression coverage for conversation loading and missing internal profiles

Why

/api/conversations/load is called by the chat store, but conversation_participants.user_id references users.id. The route was using the Supabase Auth id directly, so valid users could get an empty conversation list.

Validation

• corepack pnpm exec vitest run --config %TEMP%/qryptchat-vitest-no-react.config.mjs src/app/api/conversations/load/route.test.js
• corepack pnpm exec oxlint src/app/api/conversations/load/route.js src/app/api/conversations/load/route.test.js