fix(stream): skip non-object envelopes in onResumeStreamReplay

Matches the existing guard in clearResumeFence. A malformed entry
from the server (version skew, serialization mishap) would otherwise
throw on `envelope._replayed = true` mid-iteration and leave the
fence half-flushed via the finally-block.

src/lib/components/chat/Chat.svelte

@@ -721,6 +721,7 @@
 		const envelopes = Array.isArray(payload?.envelopes) ? payload.envelopes : [];
 		try {
 			for (const envelope of envelopes) {
+				if (!envelope || typeof envelope !== 'object') continue;
 				envelope._replayed = true;
 				await chatEventHandler(envelope);
 			}