[FE-32] Build Member Directory component with search and avatar fallback

[devwums]

Problem

There is no way for members to discover other hub members. No member directory or public profile exists, hindering community building.

Proposed Solution

Create frontend/cntr/MemberDirectory/MemberDirectory.tsx. Props: members: { id: string, fullName: string, avatarUrl?: string, role: string, joinedDate: string }[], isLoading?: boolean. Renders a responsive grid of member cards. Each card shows avatar (or initials in a colour circle), full name, role badge, and join date. Includes a client-side search input filtering by fullName. All implementation must live inside frontend/cntr/.

Acceptance Criteria

• File at frontend/cntr/MemberDirectory/MemberDirectory.tsx
• Shows skeleton cards when isLoading is true
• Search input filters members by fullName (case-insensitive)
• When avatarUrl is null, shows initials in a coloured circle (colour derived from name)
• Grid is responsive (at least 2 columns on mobile, 3–4 on desktop)
• Test in frontend/cntr/MemberDirectory/MemberDirectory.test.tsx