fix: recover null currentId on message submit

Author: Algorithm5838

src/lib/components/chat/Chat.svelte

@@ -1925,6 +1925,17 @@
 
 		// Create user message
 		let userMessageId = uuidv4();
+
+		// Recover currentId if it was unexpectedly cleared in an existing chat
+		if (!history.currentId && Object.keys(history.messages ?? {}).length > 0 && $chatId) {
+			const allParents = new Set(
+				Object.values(history.messages).map((m) => m.parentId).filter(Boolean)
+			);
+			const leaves = Object.values(history.messages).filter((m) => !allParents.has(m.id));
+			history.currentId =
+				leaves.sort((a, b) => (b.timestamp ?? 0) - (a.timestamp ?? 0))[0]?.id ?? null;
+		}
+
 		let userMessage = {
 			id: userMessageId,
 			parentId: history.currentId ?? null,