feat: user-choosable avatar colors with settings picker

Add avatarColor as a stored, user-selectable field (mint, sky, lavender,
coral, rose, teal, amber). Randomly assigned on registration, changeable
in profile settings. Existing users keep hash-based fallback until they
choose a color. Includes DB migration, API validation, gradient map,
live preview in settings, and banner fallback integration.

packages/web/src/components/modals/UserProfileModal.tsx

@@ -150,7 +150,7 @@ export function UserProfileModal() {
     : null;
   const bannerFallback = user.accentColor
     ? `linear-gradient(135deg, ${user.accentColor}, ${adjustColor(user.accentColor, -40)})`
-    : getAvatarGradient(user.homeUserId ?? user.id, displayName).gradient;
+    : getAvatarGradient(user.homeUserId ?? user.id, displayName, user.avatarColor).gradient;
 
   const handleSendMessage = async () => {
     try {