feat: let users change their auto-generated username in Settings

Usernames are auto-assigned at signup (shipped earlier this session)
but there was still no way to change one. Adds a Username field to
Settings, validated client- and server-side against the same
3-20-char lowercase/digits/underscore pattern used for generation,
with a 409 on collision (excluding the user's own current username).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/web/app/(app)/settings/page.tsx

@@ -12,7 +12,7 @@ export default async function SettingsPage() {
 
   const dbUser = await db.query.users.findFirst({
     where: eq(users.id, session.user.id),
-    columns: { bio: true, privateBio: true, isPrivate: true, hasCustomAvatar: true, avatarUrl: true },
+    columns: { bio: true, privateBio: true, isPrivate: true, hasCustomAvatar: true, avatarUrl: true, username: true },
   });
 
   return (
@@ -26,6 +26,7 @@ export default async function SettingsPage() {
         privateBio: dbUser?.privateBio ?? null,
         isPrivate: dbUser?.isPrivate ?? false,
         hasCustomAvatar: dbUser?.hasCustomAvatar ?? false,
+        username: dbUser?.username ?? null,
       }}
     />
   );