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/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