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/config/config.go

@@ -0,0 +1,87 @@
+// Package config resolves platform-specific application paths and configuration.
+package config
+
+import (
+	"os"
+	"path/filepath"
+	"runtime"
+)
+
+const appName = "NexusTerminal"
+
+// AppConfig holds the resolved application configuration paths.
+type AppConfig struct {
+	DataDir    string // base data directory
+	DBPath     string // SQLite database path
+	SecretPath string // encrypted vault directory
+	LogPath    string // log file path
+}
+
+// Load resolves the application configuration.
+// Data directory follows platform conventions:
+//   - macOS: ~/Library/Application Support/NexusTerminal
+//   - Linux: ~/.config/NexusTerminal
+//   - Windows: %APPDATA%/NexusTerminal
+func Load() (*AppConfig, error) {
+	dataDir, err := resolveDataDir()
+	if err != nil {
+		return nil, err
+	}
+
+	cfg := &AppConfig{
+		DataDir:    dataDir,
+		DBPath:     filepath.Join(dataDir, "nexus.db"),
+		SecretPath: filepath.Join(dataDir, "vault"),
+		LogPath:    filepath.Join(dataDir, "nexus.log"),
+	}
+	return cfg, nil
+}
+
+// EnsureDirectories creates required directories if they don't exist.
+func (c *AppConfig) EnsureDirectories() error {
+	dirs := []string{
+		c.DataDir,
+		c.SecretPath,
+		filepath.Dir(c.LogPath),
+	}
+	for _, d := range dirs {
+		if err := os.MkdirAll(d, 0700); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// resolveDataDir determines the base data directory for the current platform.
+func resolveDataDir() (string, error) {
+	var base string
+
+	switch runtime.GOOS {
+	case "darwin":
+		home, err := os.UserHomeDir()
+		if err != nil {
+			return "", err
+		}
+		base = filepath.Join(home, "Library", "Application Support")
+	case "windows":
+		base = os.Getenv("APPDATA")
+		if base == "" {
+			home, err := os.UserHomeDir()
+			if err != nil {
+				return "", err
+			}
+			base = filepath.Join(home, "AppData", "Roaming")
+		}
+	default: // Linux and other UNIX-likes
+		base = os.Getenv("XDG_CONFIG_HOME")
+		if base == "" {
+			home, err := os.UserHomeDir()
+			if err != nil {
+				return "", err
+			}
+			base = filepath.Join(home, ".config")
+		}
+	}
+
+	return filepath.Join(base, appName), nil
+}