Stop a non-string filter code from crashing the nfproxy page

`genericapi` ran every response body through `JSON.parse`, falling back to
the raw text only when that threw. The nfproxy filter source is served as a
PlainTextResponse, so a filter file that happens to be valid JSON came back
as a number/array/object. Mantine's CodeHighlight then calls `code.trim()`
on it, which throws and takes the whole page down with it. `42`, `"abc"`,
`[1,2]` and `{"a":1}` are all both valid JSON and valid Python, so they
pass the upload validation and leave the service page unreachable.

Responses declaring text/plain are no longer parsed, and the render site
type-checks the code before handing it to CodeHighlight.

frontend/src/js/utils.tsx

@@ -111,7 +111,11 @@ export async function genericapi(method:string,path:string,data:any = undefined,
                 const errorDefault = res.statusText
                 return res.json().then( res => reject(getErrorMessageFromServerResponse(res, errorDefault)) ).catch( _err => reject(errorDefault)) 
             }
+            // text/plain bodies (the nfproxy filter source) stay strings: a source file that
+            // happens to be valid JSON must not be parsed into a number/array/object
+            const isPlainText = (res.headers.get("content-type") ?? "").startsWith("text/plain")
             res.text().then(t => {
+                if (isPlainText) return resolve(t)
                 try{
                     resolve(JSON.parse(t))
                 }catch(e){

frontend/src/pages/NFProxy/ServiceDetails.tsx

@@ -207,7 +207,7 @@ export default function ServiceDetailsNFProxy() {
             </Group>
         </Card>
 
-        {filterCode.data ? <>
+        {typeof filterCode.data === "string" && filterCode.data ? <>
             <Group justify="center" gap="xs" mt="xl">
                 <ThemeIcon radius="md" size="md" variant="light" color="yellow"><FaPython size={16} /></ThemeIcon>
                 <Title order={3}>Filter code</Title>