feat: add unread receipts and private typing

[shakkernerd]

Summary

• add retry-safe optimistic sends with nonce replay handling
• add per-user channel/DM read receipts, unread counts, and private read events
• restrict DM typing events to direct-conversation members
• keep unread state accurate when users scroll or jump to the bottom
• expose the new read APIs and nonce support in OpenAPI, SDK, docs, and changelog
• rebuild embedded web assets

Verification

• pnpm check
• pnpm build
• diff -qr apps/web/dist apps/api/internal/webassets/dist
• git diff --check
• pnpm test:e2e -- tests/e2e/chat.spec.ts -g "sends messages" (7 passed)