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/server/src/db/schema.ts

@@ -14,6 +14,7 @@ export const users = sqliteTable('users', {
   replicatedInstances: text('replicated_instances').default('[]'),
   banner: text('banner'),
   accentColor: text('accent_color'),
+  avatarColor: text('avatar_color'),
   bio: text('bio'),
   createdAt: integer('created_at').notNull(),
 });