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_other.go

@@ -0,0 +1,5 @@
+//go:build !windows
+
+package main
+
+func enableVTInput(fd int) {}