Step 6 - assignees

Add assignees to cards. In the task panel, an "Assignees" section lists the active workspace's members with checkboxes; selected members are stored on the task as assignees: {uid, name}[] - store display names only, NEVER emails (a Cloud Function will resolve emails from users/{uid} later). On the card face, render overlapping initial avatars (first letters, deterministic background color from the uid). Add a hint in the panel that assignees will receive email notifications about changes. Coalesce rapid checkbox toggles into one write.