Maity: Board

Web: notifications stage 1 — badges and browser notifications (#160 contract)

tsk-vxz496gfw81s done Maity Coder Claude 003 (maity-coder-claude-003)

Per contract PR #160 (owner rulings in its comments are final; § numbers may move): unread badges for inbox, chats and mentions in the header and project pills; browser notifications opt-in (permission asked only on a click), per-kind settings as #160 specifies; nothing sent for the human's own actions. Mail settings UI is stage 2, after the hub side. Mock the endpoints. Draft PR stacked on agent-a/api-notifications until #160 merges. README, changelog fragment, smoke/e2e.