fix(fe): crash 'reading channel_id' + recharts width/height warnings

- useMessageDetail: guard attachments fetcher — revalidate bisa race
  detail load, detail.data undefined saat fetcher jalan ->
  TypeError 'Cannot read properties of undefined (reading channel_id)'
  yang bikin halaman /messages mati (Next error boundary). Sekarang
  fetcher balikin [] kalau channel_id belum ada; hapus non-null
  assertion. Diverifikasi: /messages sebelumnya crash, sekarang render
  dengan data asli (list, verdict, confidence, sticker, emoji).
- ResponsiveContainer (recharts 3.8): initialDimension -1 di render
  pertama -> warning 'width(-1) and height(-1)'. Pakai height numerik
  tetap (192/160/48) + minWidth/minHeight 0 -> calculatedHeight >0,
  warning hilang; width tetap responsif via ResizeObserver. Chart
  baru dirender setelah mount (useMounted) biar container punya ukuran.
  Diverifikasi console: 0 warning, 0 error di dashboard & voice.

services/frontend/src/hooks/use-messages.ts

@@ -126,14 +126,17 @@ export function useReview(channelId?: string) {
 
 export function useMessageDetail(id: string | null) {
   const detail = useSWR<MessageRecord>(id ? msgKeys.detail(id) : null, () =>
-    messagesApi.getDetail(id!),
+    messagesApi.getDetail(id ?? ""),
   );
+  const channelId = id ? detail.data?.channel_id : undefined;
   const attachments = useSWR<AttachmentRecord[]>(
-    id && detail.data?.channel_id
-      ? [...msgKeys.detail(id), "attachments"]
-      : null,
+    channelId ? [...msgKeys.detail(id ?? ""), "attachments"] : null,
     async () => {
-      const res = await messagesApi.getAttachments(detail.data!.channel_id, 10);
+      // Guard: only fetch when we actually have a channel id — a revalidate
+      // can race the detail load and see detail.data === undefined.
+      const cid = detail.data?.channel_id;
+      if (!cid) return [];
+      const res = await messagesApi.getAttachments(cid, 10);
       return res.data;
     },
   );