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/flag/flag.go

@@ -45,6 +45,10 @@ func Init(cfg *config.Config) {
 	_ = pflag.Lookup("time") // mark for NoOptDefVal
 }
 
+// Normalize converts single-dash long flags (e.g. -port) to double-dash (--port).
+// Parse wraps pflag.Parse.
+func Parse() { pflag.Parse() }
+
 // Normalize converts single-dash long flags (e.g. -port) to double-dash (--port).
 func Normalize() {
 	known := map[string]bool{