refactor: split termapp into proper internal packages

Replace monolithic internal/termapp with proper separation:
- internal/app: App struct, lifecycle, output loops
- internal/command: CommandHost interface, Dispatcher, handlers
- internal/tui: Model, hotkeys, panels, render (with panelError + border fixes)
- internal/console: RunConsole, escape parsing, entry point logic
- cmd/serialterminal: thin main() calling console.Run()

Eliminate global vars (cfg, sess, out) via dependency injection.
Break App->CommandDispatcher cycle via CommandHost interface.

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

internal/console/console_windows.go

@@ -0,0 +1,14 @@
+//go:build windows
+
+package console
+
+import "golang.org/x/sys/windows"
+
+func enableVTInput(fd int) {
+	var mode uint32
+	if err := windows.GetConsoleMode(windows.Handle(fd), &mode); err != nil {
+		return
+	}
+	mode |= windows.ENABLE_VIRTUAL_TERMINAL_INPUT
+	_ = windows.SetConsoleMode(windows.Handle(fd), mode)
+}