fix: allow null avatarUrl in updateProfileSchema

The Zod schema rejected null for avatarUrl, but the client sends null
when the avatar is removed. Changed to z.string().nullable().optional().

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/shared/schemas.ts

@@ -136,7 +136,7 @@ export const bulkUpsertGlobalItemsSchema = z.object({
 // Profile schemas
 export const updateProfileSchema = z.object({
 	displayName: z.string().max(100).optional(),
-	avatarUrl: z.string().optional(),
+	avatarUrl: z.string().nullable().optional(),
 	bio: z.string().max(500).optional(),
 });