fix(web): add random suffix to fallback assistant message IDs to prevent collisions

Message IDs generated in the fallback path used only Date.now() which caused
ID collisions in rapid-send scenarios (multiple sends within the same millisecond).
Adding the same Math.random() suffix used by userMessage IDs ensures uniqueness.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/web/src/hooks/useChat.ts

@@ -294,7 +294,7 @@ export function useChat(options: UseChatOptions = {}): UseChatReturn {
           const response = await sendChatMessage(request);
 
           const assistantMessage: Message = {
-            id: `assistant-${Date.now().toString()}`,
+            id: `assistant-${Date.now().toString()}-${Math.random().toString(36).slice(2, 8)}`,
             role: "assistant",
             content: response.message.content,
             createdAt: new Date().toISOString(),
@@ -328,7 +328,7 @@ export function useChat(options: UseChatOptions = {}): UseChatReturn {
           });
 
           const errorMessage: Message = {
-            id: `error-${String(Date.now())}`,
+            id: `error-${String(Date.now())}-${Math.random().toString(36).slice(2, 8)}`,
             role: "assistant",
             content: "Something went wrong. Please try again.",
             createdAt: new Date().toISOString(),