Add Go project scaffold for Wails v2 SSH/Telnet/RAW TCP client

- go.mod with Wails v2.9.2, x/crypto, go-sqlite3, go-keyring, uuid
- cmd/app/main.go: Wails entry point (Nexus Terminal, 1400x900, #1a1a2e)
- internal/app/app.go: lifecycle (Startup/Shutdown/DomReady)
- internal/models/models.go: full model definitions (Profile, SSHConfig,
  TelnetConfig, RAWConfig, SessionInfo, KnownHost, AppPreferences, etc.)
- internal/models/defaults.go: default values and NewProfile factory
- internal/validation/validation.go: thorough field-level validation
- frontend/embed.go: embedded frontend assets
- Updated existing storage, secrets, config to match new model structure

cmd/app/main.go

@@ -0,0 +1,39 @@
+// Nexus Terminal – a desktop SSH, Telnet and RAW TCP client built with Wails v2.
+// This is the main entry point for the application.
+package main
+
+import (
+	"log"
+
+	"github.com/wailsapp/wails/v2"
+	"github.com/wailsapp/wails/v2/pkg/options"
+	"github.com/wailsapp/wails/v2/pkg/options/assetserver"
+
+	application "github.com/copilot-ssh-client/app/internal/app"
+	"github.com/copilot-ssh-client/app/frontend"
+)
+
+func main() {
+	app := application.NewApp()
+
+	err := wails.Run(&options.App{
+		Title:     "Nexus Terminal",
+		Width:     1400,
+		Height:    900,
+		MinWidth:  1024,
+		MinHeight: 600,
+		AssetServer: &assetserver.Options{
+			Assets: frontend.Assets,
+		},
+		BackgroundColour: &options.RGBA{R: 26, G: 26, B: 46, A: 255}, // #1a1a2e
+		OnStartup:        app.Startup,
+		OnShutdown:       app.Shutdown,
+		OnDomReady:       app.DomReady,
+		Bind: []interface{}{
+			app,
+		},
+	})
+	if err != nil {
+		log.Fatalf("failed to start Nexus Terminal: %v", err)
+	}
+}