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/preferences/preferences.go

@@ -0,0 +1,107 @@
+// Package preferences manages application preferences with in-memory caching
+// and persistence to a backing store.
+package preferences
+
+import (
+	"fmt"
+	"sync"
+
+	"github.com/copilot-ssh-client/app/internal/models"
+)
+
+// PreferenceStore is the persistence interface used by the Manager.
+type PreferenceStore interface {
+	GetPreferences() (*models.AppPreferences, error)
+	SavePreferences(prefs *models.AppPreferences) error
+}
+
+// Manager handles application preferences with defaults and caching.
+// It is safe for concurrent use.
+type Manager struct {
+	store PreferenceStore
+	cache *models.AppPreferences
+	mu    sync.RWMutex
+}
+
+// NewManager creates a preferences Manager backed by the given store.
+// Call Load to populate the cache from disc.
+func NewManager(store PreferenceStore) *Manager {
+	return &Manager{store: store}
+}
+
+// Load reads preferences from the backing store into memory. If no
+// preferences have been saved yet, sensible defaults are applied.
+func (m *Manager) Load() error {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	prefs, err := m.store.GetPreferences()
+	if err != nil {
+		return fmt.Errorf("preferences: failed to load: %w", err)
+	}
+
+	if prefs == nil {
+		defaults := models.DefaultAppPreferences()
+		m.cache = &defaults
+	} else {
+		m.cache = prefs
+		m.applyDefaults()
+	}
+	return nil
+}
+
+// Get returns the current preferences. If Load has not been called, defaults
+// are returned.
+func (m *Manager) Get() *models.AppPreferences {
+	m.mu.RLock()
+	defer m.mu.RUnlock()
+
+	if m.cache == nil {
+		defaults := models.DefaultAppPreferences()
+		return &defaults
+	}
+	// Return a copy to prevent callers mutating the cache.
+	cp := *m.cache
+	return &cp
+}
+
+// Save persists the given preferences to the backing store and updates the
+// in-memory cache.
+func (m *Manager) Save(prefs *models.AppPreferences) error {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	if err := m.store.SavePreferences(prefs); err != nil {
+		return fmt.Errorf("preferences: failed to save: %w", err)
+	}
+	cp := *prefs
+	m.cache = &cp
+	return nil
+}
+
+// applyDefaults fills in zero-valued fields from the default preferences.
+// Caller must hold m.mu.
+func (m *Manager) applyDefaults() {
+	defaults := models.DefaultAppPreferences()
+	if m.cache.Theme == "" {
+		m.cache.Theme = defaults.Theme
+	}
+	if m.cache.DefaultProtocol == "" {
+		m.cache.DefaultProtocol = defaults.DefaultProtocol
+	}
+	if m.cache.SidebarWidth == 0 {
+		m.cache.SidebarWidth = defaults.SidebarWidth
+	}
+	if m.cache.DefaultFontFamily == "" {
+		m.cache.DefaultFontFamily = defaults.DefaultFontFamily
+	}
+	if m.cache.DefaultFontSize == 0 {
+		m.cache.DefaultFontSize = defaults.DefaultFontSize
+	}
+	if m.cache.DefaultScrollback == 0 {
+		m.cache.DefaultScrollback = defaults.DefaultScrollback
+	}
+	if m.cache.DefaultCursorStyle == "" {
+		m.cache.DefaultCursorStyle = defaults.DefaultCursorStyle
+	}
+}