refactor: extract pkg/charset and internal/event packages

Extract ConvertChunk/FormatHexFrame into pkg/charset (zero external deps).
Extract UIEvent/UIEventKind/UIPanelKind types into internal/event.
Update all references across main package to use qualified imports.

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

main_windows.go

@@ -0,0 +1,14 @@
+//go:build windows
+
+package main
+
+import (
+	"golang.org/x/sys/windows"
+)
+
+func enableVTInput(fd int) {
+	var mode uint32
+	if err := windows.GetConsoleMode(windows.Handle(fd), &mode); err == nil {
+		_ = windows.SetConsoleMode(windows.Handle(fd), mode|windows.ENABLE_VIRTUAL_TERMINAL_INPUT)
+	}
+}