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_other.go

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