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/config.go

@@ -0,0 +1,10 @@
+package termapp
+
+import (
+	appconfig "github.com/jixishi/SerialTerminalForWindowsTerminal/internal/config"
+)
+
+// Config is an alias for appconfig.Config to keep main-package code concise.
+type Config = appconfig.Config
+
+var cfg = &Config{}