Feature: Add avatar color and initials generator #177

[wolfyres]

Closes #177

🎯 Context

This PR introduces utilities to generate consistent sender avatar colors and initials for demo personas. To ensure zero risk to the core product, these utilities and their preview components have been strictly isolated to the demo admin tooling, bypassing all production app-shell and UI components.

🛠️ Key Changes

Implemented getAvatarColor as a deterministic seed helper to hash a string (like a sender name) into a consistent hex color from a predefined palette.

Built the AvatarPreview component to allow dashboard maintainers to easily visualize and verify the generated avatars for fake personas.

Added a robust local test suite (avatar-helpers.test.ts) to guarantee the color hashing and initial extraction remain completely deterministic.

Confirmed all files are scoped exclusively to src/features/demo-admin-dashboard/.