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/utils/sanitize.ts

@@ -18,6 +18,7 @@ export function sanitizeUser(row: typeof schema.users.$inferSelect): User {
     avatar: row.avatar,
     banner: row.banner ?? null,
     accentColor: row.accentColor ?? null,
+    avatarColor: (row.avatarColor as User['avatarColor']) ?? null,
     bio: row.bio ?? null,
     status: (row.status ?? 'offline') as User['status'],
     customStatus: row.customStatus,