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/lib/hooks/use-mounted.ts

@@ -0,0 +1,16 @@
+import { useEffect, useState } from "react";
+
+/**
+ * Returns true once the component has mounted on the client. Charts wrapped
+ * in ResponsiveContainer measure their parent during the first layout — if
+ * the container has no size yet (hydration/flex/grid), Recharts logs
+ * "width(-1) and height(-1)" warnings. Deferring the chart render until after
+ * mount guarantees the container has real dimensions.
+ */
+export function useMounted(): boolean {
+  const [mounted, setMounted] = useState(false);
+  useEffect(() => {
+    setMounted(true);
+  }, []);
+  return mounted;
+}