feat: add config, auth, fetcher, java modules

- config: OS-specific paths, launcher.json load/save, DefaultSettings with MRIXSCRAFT_SERVER_URL env override
- auth: Yggdrasil client (authenticate, refresh, validate), session persistence in session.json, EnsureValid flow
- fetcher: HTTP download with SHA-1 verification, WorkerPool for concurrent downloads
- java: JRE detection (IsInstalled/Find), platform-specific executable name
- utils: SHA1File, SHA1Bytes, Unzip with zip-slip protection
- cmd/launcher: wire config + auth into main, session restore on startup

Co-Authored-By: OWL <noreply@anthropic.com>

internal/config/config.go

@@ -1,2 +1,172 @@
-// package config manages launcher configuration (launcher.json, system paths).
+// package config manages launcher configuration and system paths.
 package config
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"runtime"
+)
+
+// AppName is the canonical name for the launcher and its data directory.
+const AppName = "MrixsCraft"
+
+// Settings represents the user-configurable launcher settings stored in launcher.json.
+type Settings struct {
+	ServerURL    string `json:"server_url"`    // Base URL of the backend (e.g. "https://minecraft.mrixs.me")
+	SelectedPack string `json:"selected_pack"` // Slug of the selected modpack
+	MemoryMB     int    `json:"memory_mb"`     // Allocated RAM in MB (for -Xms / -Xmx)
+	ExtraArgs    string `json:"extra_args"`    // Additional JVM flags
+	Width        int    `json:"window_width"`  // Main window width
+	Height       int    `json:"window_height"` // Main window height
+}
+
+// serverEnvURL is the environment variable that overrides the backend base URL.
+const serverEnvURL = "MRIXSCRAFT_SERVER_URL"
+
+// defaultServerURL is used when neither the env var nor launcher.json provide a value.
+const defaultServerURL = "https://minecraft.mrixs.me"
+
+// DefaultSettings returns sensible defaults.
+func DefaultSettings() Settings {
+	url := os.Getenv(serverEnvURL)
+	if url == "" {
+		url = defaultServerURL
+	}
+	return Settings{
+		ServerURL:    url,
+		SelectedPack: "",
+		MemoryMB:     4096,
+		ExtraArgs:    "",
+		Width:        900,
+		Height:       600,
+	}
+}
+
+// RootDir returns the OS-specific data directory for the launcher.
+func RootDir() (string, error) {
+	switch runtime.GOOS {
+	case "windows":
+		appData := os.Getenv("APPDATA")
+		if appData == "" {
+			return "", fmt.Errorf("APPDATA environment variable is empty")
+		}
+		return filepath.Join(appData, AppName), nil
+
+	case "darwin":
+		home, err := os.UserHomeDir()
+		if err != nil {
+			return "", fmt.Errorf("resolving home directory: %w", err)
+		}
+		return filepath.Join(home, "Library", "Application Support", AppName), nil
+
+	default: // linux and other unixes
+		home, err := os.UserHomeDir()
+		if err != nil {
+			return "", fmt.Errorf("resolving home directory: %w", err)
+		}
+		return filepath.Join(home, "."+AppName), nil
+	}
+}
+
+// EnsureRoot creates the root data directory if it does not exist.
+func EnsureRoot() (string, error) {
+	dir, err := RootDir()
+	if err != nil {
+		return "", err
+	}
+	if err := os.MkdirAll(dir, 0o755); err != nil {
+		return "", fmt.Errorf("creating root directory %s: %w", dir, err)
+	}
+	return dir, nil
+}
+
+// Subdirectory returns (and optionally creates) a subdirectory under the root.
+func Subdirectory(name string, create bool) (string, error) {
+	root, err := RootDir()
+	if err != nil {
+		return "", err
+	}
+	path := filepath.Join(root, name)
+	if create {
+		if err := os.MkdirAll(path, 0o755); err != nil {
+			return "", fmt.Errorf("creating subdirectory %s: %w", path, err)
+		}
+	}
+	return path, nil
+}
+
+// JavaDir returns the directory for a specific Java version.
+func JavaDir(version int) (string, error) {
+	return Subdirectory(filepath.Join("Java", fmt.Sprintf("%d", version)), true)
+}
+
+// InstancesDir returns the directory containing modpack instances.
+func InstancesDir() (string, error) {
+	return Subdirectory("instances", true)
+}
+
+// InstanceDir returns the directory for a specific modpack.
+func InstanceDir(slug string) (string, error) {
+	return Subdirectory(filepath.Join("instances", slug), true)
+}
+
+// AssetsDir returns the assets cache directory.
+func AssetsDir() (string, error) {
+	return Subdirectory("assets", true)
+}
+
+// LibrariesDir returns the libraries cache directory.
+func LibrariesDir() (string, error) {
+	return Subdirectory("libraries", true)
+}
+
+// LauncherJSONPath returns the full path to launcher.json.
+func LauncherJSONPath() (string, error) {
+	root, err := RootDir()
+	if err != nil {
+		return "", err
+	}
+	return filepath.Join(root, "launcher.json"), nil
+}
+
+// Load reads launcher.json from disk, returning defaults if the file does not exist.
+func Load() (Settings, error) {
+	path, err := LauncherJSONPath()
+	if err != nil {
+		return Settings{}, err
+	}
+
+	data, err := os.ReadFile(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return DefaultSettings(), nil
+		}
+		return Settings{}, fmt.Errorf("reading %s: %w", path, err)
+	}
+
+	var s Settings
+	if err := json.Unmarshal(data, &s); err != nil {
+		return Settings{}, fmt.Errorf("parsing %s: %w", path, err)
+	}
+	return s, nil
+}
+
+// Save writes the settings to launcher.json.
+func Save(s Settings) error {
+	path, err := LauncherJSONPath()
+	if err != nil {
+		return err
+	}
+
+	data, err := json.MarshalIndent(s, "", "  ")
+	if err != nil {
+		return fmt.Errorf("serializing settings: %w", err)
+	}
+
+	if err := os.WriteFile(path, data, 0o600); err != nil {
+		return fmt.Errorf("writing %s: %w", path, err)
+	}
+	return nil
+}