feat: fix Ctrl+J newline in TUI

cmd/sidekick-tui/main.go

@@ -161,11 +161,14 @@ func (m model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
     vpWidth := msg.Width - 4 // Account for viewportStyle padding and border
     vpHeight := msg.Height - headerHeight - inputHeight - 2
 
-    r, _ := glamour.NewTermRenderer(
-      glamour.WithAutoStyle(),
+    if vpWidth > 0 && vpHeight > 0 {
+      r, err := glamour.NewTermRenderer(
+        glamour.WithStandardStyle("dark"),
         glamour.WithWordWrap(vpWidth-2),
       )
+      if err == nil {
         m.renderer = r
+      }
 
       if !m.ready {
         m.viewport = viewport.New(vpWidth, vpHeight)
@@ -176,6 +179,7 @@ func (m model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
       }
       m.updateViewportContent()
       m.viewport.GotoBottom()
+    }
     
     m.textarea.SetWidth(msg.Width - 4)
 
@@ -189,6 +193,9 @@ func (m model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
     case tea.KeyPgDown, tea.KeyCtrlD:
       m.viewport.HalfViewDown()
       return m, nil
+    case tea.KeyCtrlJ:
+      m.textarea.InsertString("\n")
+      return m, nil
     case tea.KeyEnter:
       v := m.textarea.Value()
       if strings.TrimSpace(v) == "" || m.isThinking {