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.

internal/acp/client_test.go

@@ -276,3 +276,78 @@ func TestAgentRequestReadFile(t *testing.T) {
 		t.Fatalf("content = %q, want 'package main\\n'", res.Content)
 	}
 }
+
+func TestHistoryAppendOrCreateGroupsByMessageID(t *testing.T) {
+	h := NewHistory()
+
+	// 1. First chunk for a new messageId creates an entry.
+	time.Sleep(10 * time.Millisecond)
+	beforeFirst := time.Now()
+	time.Sleep(10 * time.Millisecond)
+	firstTime := h.AppendOrCreate("agent", "msg-1", "hello")
+	time.Sleep(10 * time.Millisecond)
+	afterFirst := time.Now()
+
+	if firstTime.Before(beforeFirst) || firstTime.After(afterFirst) {
+		t.Fatalf("first chunk timestamp %v out of range [%v, %v]", firstTime, beforeFirst, afterFirst)
+	}
+	msgs := h.List()
+	if len(msgs) != 1 {
+		t.Fatalf("after first chunk: len = %d, want 1", len(msgs))
+	}
+	if msgs[0].Role != "agent" || msgs[0].Text != "hello" || msgs[0].MessageID != "msg-1" {
+		t.Fatalf("first chunk message = %+v, want role=agent text=hello messageId=msg-1", msgs[0])
+	}
+
+	// 2. Second chunk for the same messageId appends and preserves timestamp.
+	time.Sleep(10 * time.Millisecond)
+	secondTime := h.AppendOrCreate("agent", "msg-1", " world")
+	if !secondTime.Equal(firstTime) {
+		t.Fatalf("second chunk timestamp changed: got %v, want %v", secondTime, firstTime)
+	}
+	msgs = h.List()
+	if len(msgs) != 1 {
+		t.Fatalf("after second chunk: len = %d, want 1", len(msgs))
+	}
+	if msgs[0].Text != "hello world" {
+		t.Fatalf("after second chunk: text = %q, want \"hello world\"", msgs[0].Text)
+	}
+
+	// 3. A different messageId creates a new entry.
+	h.AppendOrCreate("agent", "msg-2", "second")
+	msgs = h.List()
+	if len(msgs) != 2 {
+		t.Fatalf("after different id: len = %d, want 2", len(msgs))
+	}
+	if msgs[1].Text != "second" || msgs[1].MessageID != "msg-2" {
+		t.Fatalf("second message = %+v, want text=second messageId=msg-2", msgs[1])
+	}
+
+	// 4. Empty messageId always appends a new entry and never collides.
+	h.AppendOrCreate("user", "", "a")
+	h.AppendOrCreate("user", "", "b")
+	msgs = h.List()
+	if len(msgs) != 4 {
+		t.Fatalf("after two empty-id chunks: len = %d, want 4", len(msgs))
+	}
+	if msgs[2].Text != "a" || msgs[2].MessageID != "" {
+		t.Fatalf("empty-id message[2] = %+v, want text=a messageId=\"\"", msgs[2])
+	}
+	if msgs[3].Text != "b" || msgs[3].MessageID != "" {
+		t.Fatalf("empty-id message[3] = %+v, want text=b messageId=\"\"", msgs[3])
+	}
+
+	// 5. Clear resets the index map so the same id can be reused.
+	h.Clear()
+	if h.Len() != 0 {
+		t.Fatalf("after Clear: len = %d, want 0", h.Len())
+	}
+	h.AppendOrCreate("agent", "msg-1", "after clear")
+	msgs = h.List()
+	if len(msgs) != 1 {
+		t.Fatalf("after clear + chunk: len = %d, want 1", len(msgs))
+	}
+	if msgs[0].Text != "after clear" || msgs[0].MessageID != "msg-1" {
+		t.Fatalf("after clear message = %+v, want text=\"after clear\" messageId=msg-1", msgs[0])
+	}
+}