refactor: move all app code to internal/termapp and entry to cmd/

Consolidate remaining package main files into internal/termapp for
clean separation: cmd/serialterminal/ is the sole entry point.
Update .goreleaser.yaml to build from ./cmd/serialterminal.

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

internal/termapp/main_windows.go

@@ -0,0 +1,14 @@
+//go:build windows
+
+package termapp
+
+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)
+	}
+}