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/migrate.ts

@@ -93,6 +93,12 @@ export function runMigrations(db: Database.Database): void {
         { name: 'accent_color', type: 'TEXT' },
         { name: 'bio', type: 'TEXT' },
       ]
+    },
+    {
+      name: 'users',
+      columns: [
+        { name: 'avatar_color', type: 'TEXT' },
+      ]
     }
   ];