fix: preserve full content in scrollback by separating render cache from viewport

The StreamComponent was truncating content to fit the viewport height before
caching it in renderCache. This caused GetRenderedContent() to return truncated
content when flushing to scrollback.

Changes:
- render() now caches FULL content without height clamping
- New viewContent() helper applies height clamping only for display
- View() calls both: render() for full content, viewContent() for visible slice

This follows the Pi TUI pattern: full buffer in memory, viewport slicing only
at display time. Long assistant messages are now fully preserved in scrollback.

internal/ui/stream.go

@@ -282,7 +282,8 @@ func (s *StreamComponent) GetRenderedContent() string {
 
 	text := s.streamContent.String()
 	if text != "" {
-		sections = append(sections, s.renderStreamingText(text))
+		rendered := s.renderStreamingText(text)
+		sections = append(sections, rendered)
 	}
 
 	if len(sections) == 0 {
@@ -415,7 +416,9 @@ func (s *StreamComponent) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 
 // View implements tea.Model. Renders the current stream region content.
 func (s *StreamComponent) View() tea.View {
-	return tea.NewView(s.render())
+	fullContent := s.render()
+	visibleContent := s.viewContent(fullContent)
+	return tea.NewView(visibleContent)
 }
 
 // --------------------------------------------------------------------------
@@ -458,21 +461,27 @@ func (s *StreamComponent) render() string {
 
 	content := strings.Join(sections, "\n")
 
-	// Clamp to height if constrained: keep the last h lines so the most
-	// recent output is always visible.
-	if s.height > 0 && content != "" {
-		lines := strings.Split(content, "\n")
-		if len(lines) > s.height {
-			lines = lines[len(lines)-s.height:]
-			content = strings.Join(lines, "\n")
-		}
-	}
-
+	// Cache FULL content without height clamping.
+	// Height clamping is applied in View() for display only.
 	s.renderCache = content
 	s.renderDirty = false
 	return content
 }
 
+// viewContent returns the visible portion of content based on height constraint.
+// This is called by View() to get the slice that fits in the terminal.
+func (s *StreamComponent) viewContent(fullContent string) string {
+	if s.height > 0 && fullContent != "" {
+		lines := strings.Split(fullContent, "\n")
+		if len(lines) > s.height {
+			// Keep only the last h lines so the most recent output is visible.
+			lines = lines[len(lines)-s.height:]
+			return strings.Join(lines, "\n")
+		}
+	}
+	return fullContent
+}
+
 // renderReasoningBlock renders the reasoning/thinking content in a surface-tinted
 // box. When collapsed, shows the last 10 lines with a truncation hint. When
 // expanded, shows all lines. Includes a "Thought for Xs" duration footer.