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_model.go

@@ -16,9 +16,10 @@ import (
 type doneMsg struct{}
 
 type modeItem struct {
-	key   string
-	label string
-	value string
+	key      string
+	label    string
+	value    string
+	rawValue string
 }
 
 type panelLine struct {