fix: TUI 'g' key input, EndStr panel editing, and mode validation

- Remove 'g' and 'shift+g' from viewport handler (conflicted with
  text input; home/end keys already provide same functionality)
- Add rawValue field to modeItem to preserve EndStr control chars
  during TUI panel editing (fixes %q round-trip corruption)
- Add validation for empty charset names and timefmt fields

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/termapp/command.go

@@ -439,8 +439,14 @@ func (d *CommandDispatcher) handleModeCommand(args []string) error {
 
 	switch field {
 	case "in":
+		if value == "" {
+			return fmt.Errorf("input charset must not be empty")
+		}
 		d.app.cfg.InputCode = value
 	case "out":
+		if value == "" {
+			return fmt.Errorf("output charset must not be empty")
+		}
 		d.app.cfg.OutputCode = value
 	case "end":
 		d.app.cfg.EndStr = value
@@ -457,6 +463,9 @@ func (d *CommandDispatcher) handleModeCommand(args []string) error {
 		}
 		d.app.cfg.TimesTamp = enabled
 	case "timefmt":
+		if value == "" && d.app.cfg.TimesTamp {
+			return fmt.Errorf("timestamp format must not be empty")
+		}
 		d.app.cfg.TimesFmt = value
 	default:
 		return fmt.Errorf("unknown mode field: %s", field)

internal/termapp/tui_hotkeys.go

@@ -101,11 +101,11 @@ func (m *uiModel) handleViewportKey(msg tea.KeyMsg) bool {
 		m.viewport, cmd = m.viewport.Update(msg)
 		_ = cmd
 		return true
-	case "home", "g":
+	case "home":
 		m.viewport.GotoTop()
 		m.followTail = false
 		return true
-	case "end", "shift+g":
+	case "end":
 		m.viewport.GotoBottom()
 		m.followTail = true
 		return true

internal/termapp/tui_model.go

@@ -19,6 +19,7 @@ type modeItem struct {
 	key      string
 	label    string
 	value    string
+	rawValue string
 }
 
 type panelLine struct {

internal/termapp/tui_panels.go

@@ -71,7 +71,14 @@ func (m *uiModel) refreshPanel() {
 }
 
 func (m *uiModel) buildModeItems() []modeItem {
-	return []modeItem{{"in", "Input Charset", m.app.cfg.InputCode}, {"out", "Output Charset", m.app.cfg.OutputCode}, {"end", "Line End", fmt.Sprintf("%q", m.app.cfg.EndStr)}, {"frame", "Hex Frame Size", fmt.Sprintf("%d", m.app.cfg.FrameSize)}, {"timestamp", "Timestamp", fmt.Sprintf("%v", m.app.cfg.TimesTamp)}, {"timefmt", "Timestamp Format", m.app.cfg.TimesFmt}}
+	return []modeItem{
+		{"in", "Input Charset", m.app.cfg.InputCode, m.app.cfg.InputCode},
+		{"out", "Output Charset", m.app.cfg.OutputCode, m.app.cfg.OutputCode},
+		{"end", "Line End", fmt.Sprintf("%q", m.app.cfg.EndStr), m.app.cfg.EndStr},
+		{"frame", "Hex Frame Size", fmt.Sprintf("%d", m.app.cfg.FrameSize), fmt.Sprintf("%d", m.app.cfg.FrameSize)},
+		{"timestamp", "Timestamp", fmt.Sprintf("%v", m.app.cfg.TimesTamp), fmt.Sprintf("%v", m.app.cfg.TimesTamp)},
+		{"timefmt", "Timestamp Format", m.app.cfg.TimesFmt, m.app.cfg.TimesFmt},
+	}
 }
 
 func (m *uiModel) handleForwardPanelKey(key string) bool {
@@ -222,7 +229,7 @@ func (m *uiModel) handleModePanelKey(key string) bool {
 		}
 		return true
 	case "enter", "e":
-		initial := strings.Trim(sel.value, "\"")
+		initial := sel.rawValue
 		m.startPrompt("Edit Mode: "+sel.label, "new value", initial, func(v string) {
 			m.app.handleLine(fmt.Sprintf(".mode set %s %s", sel.key, v))
 			m.refreshPanel()