fix(acp): group agent chunks by messageId (backend) and coalesce consecutive same-role entries in the UI (frontend)

The opencode acp binary streams agent_message_chunk notifications with
an EMPTY messageId. The backend was already correct (it groups when a
messageId is present), but the UI rendered every chunk as its own
bubble, so a single 'count 1 to 5' reply showed as 9 separate bubbles.

Backend:
- internal/acp/history.go: add MessageID to Message; new AppendOrCreate
  method that finds or creates an entry by messageId and appends text
  to it (preserving the original timestamp).
- internal/acp/client.go: agent_message_chunk and user_message_chunk
  use AppendOrCreate.
- internal/model/acp.go: expose MessageID in the wire response.
- internal/service/acp_service.go: copy MessageID into the API DTO.
- internal/acp/client_test.go: add TestHistoryAppendOrCreateGroupsByMessageID.

Frontend:
- web/src/components/acp/groupMessages.ts: new pure helper that
  coalesces consecutive same-role history entries into a single bubble
  (keeps the first chunk's time + messageId).
- web/src/components/acp/AcpPanel.tsx: render coalesced messages;
  auto-scroll effect now depends on coalesced so it fires as the
  agent streams.

Verified via e2e: 'count from 1 to 5' now returns a single agent entry
with text '1\n2\n3\n4\n5' instead of 9 separate entries.

web/src/components/acp/groupMessages.ts

@@ -0,0 +1,29 @@
+import type { AcpMessage } from "@/lib/api/acp";
+
+export interface CoalescedMessage {
+  role: "user" | "agent";
+  text: string;
+  time: string;
+  messageId?: string;
+}
+
+export function groupMessages(messages: AcpMessage[]): CoalescedMessage[] {
+  const result: CoalescedMessage[] = [];
+
+  for (const message of messages) {
+    const last = result[result.length - 1];
+
+    if (last && last.role === message.role) {
+      last.text += message.text;
+    } else {
+      result.push({
+        role: message.role,
+        text: message.text,
+        time: message.time,
+        messageId: (message as AcpMessage & { messageId?: string }).messageId,
+      });
+    }
+  }
+
+  return result;
+}