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>

cmd/serialterminal/main.go

@@ -0,0 +1,7 @@
+package main
+
+import "github.com/jixishi/SerialTerminalForWindowsTerminal/internal/termapp"
+
+func main() {
+	termapp.Run()
+}