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

internal/app/app.go

@@ -0,0 +1,117 @@
+// Package app provides the core application lifecycle for Nexus Terminal.
+// It wires together storage, secrets, sessions, preferences, known hosts,
+// and the audit log, and exposes Wails lifecycle hooks.
+package app
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"sync"
+
+	"github.com/wailsapp/wails/v2/pkg/runtime"
+
+	"github.com/copilot-ssh-client/app/internal/config"
+	"github.com/copilot-ssh-client/app/internal/secrets"
+)
+
+// Storage abstracts the persistent data layer (SQLite).
+type Storage interface {
+	Close() error
+}
+
+// SessionManager tracks active connection sessions.
+type SessionManager interface {
+	CloseAll()
+}
+
+// Preferences manages user application preferences.
+type Preferences interface{}
+
+// KnownHostStore manages trusted SSH host key records.
+type KnownHostStore interface{}
+
+// AuditLog records auditable application events.
+type AuditLog interface{}
+
+// App is the central application struct. Wails binds its lifecycle methods
+// (Startup, Shutdown, DomReady) and it holds references to every subsystem.
+type App struct {
+	ctx context.Context
+	mu  sync.Mutex
+
+	Config         *config.AppConfig
+	Storage        Storage
+	SecretStore    *secrets.SecretStore
+	SessionManager SessionManager
+	Preferences    Preferences
+	KnownHostStore KnownHostStore
+	AuditLog       AuditLog
+}
+
+// NewApp creates a new App instance. Subsystems are not initialised until
+// Startup is called by the Wails runtime.
+func NewApp() *App {
+	return &App{}
+}
+
+// Startup is called by Wails when the application starts. It loads
+// configuration, ensures required directories exist, and initialises all
+// subsystems (storage, secret store, session manager, preferences).
+func (a *App) Startup(ctx context.Context) {
+	a.mu.Lock()
+	defer a.mu.Unlock()
+
+	a.ctx = ctx
+
+	cfg, err := config.Load()
+	if err != nil {
+		log.Fatalf("failed to load configuration: %v", err)
+	}
+	a.Config = cfg
+
+	if err := cfg.EnsureDirectories(); err != nil {
+		log.Fatalf("failed to create application directories: %v", err)
+	}
+
+	// Initialise the encrypted secret store.
+	a.SecretStore = secrets.NewSecretStore(cfg.SecretPath)
+
+	// Storage, session manager, preferences, known host store and audit log
+	// will be wired in when their implementations are available.
+	// For now we log successful initialisation.
+	log.Printf("Nexus Terminal initialised – data directory: %s", cfg.DataDir)
+}
+
+// Shutdown is called by Wails when the application is closing. It ensures all
+// sessions are cleanly terminated and persistent storage is flushed.
+func (a *App) Shutdown(ctx context.Context) {
+	a.mu.Lock()
+	defer a.mu.Unlock()
+
+	if a.SessionManager != nil {
+		a.SessionManager.CloseAll()
+	}
+
+	if a.Storage != nil {
+		if err := a.Storage.Close(); err != nil {
+			log.Printf("warning: error closing storage: %v", err)
+		}
+	}
+
+	// Lock the secret store to zero key material.
+	if a.SecretStore != nil {
+		a.SecretStore.Lock()
+	}
+
+	log.Println("Nexus Terminal shut down cleanly")
+}
+
+// DomReady is called by Wails once the frontend DOM is fully loaded. It emits
+// a "backend:ready" event so the frontend knows the backend is prepared.
+func (a *App) DomReady(ctx context.Context) {
+	runtime.EventsEmit(ctx, "backend:ready", map[string]interface{}{
+		"message": "Backend is ready",
+	})
+	fmt.Println("DOM ready – backend:ready event emitted")
+}