Reduce idle screenshot churn with change-driven SSE

Emit dashboard activity updates only when terminal screen content actually changes, and tighten screenshot 304 fast-path behavior to avoid stale not-modified responses after newer activity.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

go/internal/terminalstate/tracker.go

@@ -50,6 +50,7 @@ type Tracker struct {
 	screen              *te.DiffScreen
 	stream              *te.ByteStream
 	changeCounter       uint64
+	lastActivityCounter uint64
 	lastSnapshotCounter uint64
 }
 
@@ -125,6 +126,16 @@ func (t *Tracker) Snapshot() Snapshot {
 	return snapshot
 }
 
+func (t *Tracker) ConsumeActivityChanged() bool {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	if t.changeCounter > t.lastActivityCounter {
+		t.lastActivityCounter = t.changeCounter
+		return true
+	}
+	return false
+}
+
 func colorToString(color te.Color) string {
 	if color.Name != "" {
 		name := strings.ToLower(strings.TrimPrefix(color.Name, "#"))