feat(users): build users management page

[mijinummi]

Closes #460

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Users Management Page (#460)

Objective

• Build a page at /users listing all organisation members with filtering and role management.

Changes

• Added API client (usersApi.ts) for GET/PATCH endpoints.
• Added React Query hooks (useUsers, useUpdateUserRole).
• Added Avatar component with initials and current user badge.
• Implemented /users page with:
  • Table (Avatar + Name, Email, Role, Joined date).
  • Search input (filter by name/email).
  • Role filter dropdown.
  • Inline role dropdown per row (disabled for current user).
  • Role legend with color coding.

Acceptance Criteria

• ✅ Role dropdown disabled for current user.
• ✅ Role change updates immediately (no reload).
• ✅ Search and role filter can be combined.
• ✅ Avatar shows initials, darker background for current user.
• ✅ Role legend displayed.

[vercel]

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