feat: SSE Last-Event-ID 断线重放 + GUI 表单防刷新

- webui handler: 新增 sseEventRing 环状缓冲区(200条)，断线重连按 Last-Event-ID 重放遗漏事件
- GUI app.js: doRenderAll 检测连接表单打开时改走 refreshDataOnly，修复 15s 定时器擦掉用户输入的 bug
- 附带 dashboard.html/index.html 前端调整 + handler_sse_test.go 单测

internal/plugins/webui/handler_sse_test.go

@@ -0,0 +1,96 @@
+package webui
+
+import (
+	"encoding/json"
+	"reflect"
+	"testing"
+)
+
+func TestSSEEventRing_Append_Capacity(t *testing.T) {
+	r := newSSEEventRing(3)
+	data := json.RawMessage(`{"a":1}`)
+	for i := 0; i < 5; i++ {
+		r.Append("id-"+string(rune('0'+i)), "agent_output", data)
+	}
+	// cap=3，只保留 id-2,id-3,id-4
+	if len(r.buf) != 3 {
+		t.Fatalf("expected 3, got %d", len(r.buf))
+	}
+	if r.buf[0].id != "id-2" {
+		t.Fatalf("expected id-2, got %s", r.buf[0].id)
+	}
+}
+
+func TestSSEEventRing_After_Mid(t *testing.T) {
+	r := newSSEEventRing(10)
+	data := json.RawMessage(`{"x":"y"}`)
+	r.Append("id-1", "reasoning", data)
+	r.Append("id-2", "agent_output", data)
+	r.Append("id-3", "tool_call", data)
+
+	after := r.After("id-2")
+	if len(after) != 1 || after[0].id != "id-3" {
+		t.Fatalf("expected [id-3], got %v", after)
+	}
+
+	after = r.After("id-3")
+	if len(after) != 0 {
+		t.Fatalf("expected 0, got %d", len(after))
+	}
+}
+
+func TestSSEEventRing_After_NotFound_ReturnsAll(t *testing.T) {
+	r := newSSEEventRing(3)
+	data := json.RawMessage(`{}`)
+	r.Append("id-1", "agent_output", data)
+	r.Append("id-2", "agent_output", data)
+
+	after := r.After("id-0")
+	if len(after) != 2 {
+		t.Fatalf("expected 2 (fallback to all), got %d", len(after))
+	}
+}
+
+func TestSSEEventRing_After_Last_Tip(t *testing.T) {
+	r := newSSEEventRing(5)
+	data := json.RawMessage(`{"a":1}`)
+	for i := 0; i < 5; i++ {
+		r.Append("id-"+string(rune('0'+i)), "agent_output", data)
+	}
+	after := r.After("id-4")
+	if len(after) != 0 {
+		t.Fatalf("expected 0 after tip, got %d", len(after))
+	}
+}
+
+func TestSSEEventRing_Concurrent(t *testing.T) {
+	r := newSSEEventRing(100)
+	done := make(chan struct{})
+	for i := 0; i < 8; i++ {
+		go func(n int) {
+			defer func() { done <- struct{}{} }()
+			data := json.RawMessage(`{}`)
+			for j := 0; j < 100; j++ {
+				r.Append("w"+string(rune('0'+n))+"/"+string(rune('0'+j)), "agent_output", data)
+			}
+		}(i)
+	}
+	for i := 0; i < 8; i++ {
+		go func() {
+			defer func() { done <- struct{}{} }()
+			for j := 0; j < 100; j++ {
+				_ = r.After("w0/0")
+			}
+		}()
+	}
+	for i := 0; i < 16; i++ {
+		<-done
+	}
+}
+
+func TestSSEEventRecord_Type(t *testing.T) {
+	var got interface{} = sseEventRecord{}
+	if reflect.TypeOf(got).Kind() != reflect.Struct {
+		t.Fatal("sseEventRecord should be a struct")
+	}
+}