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/java/java.go

@@ -1,2 +1,64 @@
 // package java manages portable JRE downloads and detection.
 package java
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"runtime"
+
+	"gitea.mrixs.me/Mrixs/MrixsCraft-launcher/internal/config"
+)
+
+// ExecutableName returns the platform-specific Java executable name.
+func ExecutableName() string {
+	if runtime.GOOS == "windows" {
+		return "java.exe"
+	}
+	return "java"
+}
+
+// IsInstalled checks whether the given Java version is available locally.
+func IsInstalled(version int) (string, error) {
+	dir, err := config.JavaDir(version)
+	if err != nil {
+		return "", err
+	}
+	bin := filepath.Join(dir, "bin", ExecutableName())
+	if _, err := os.Stat(bin); err != nil {
+		if os.IsNotExist(err) {
+			return "", nil // not installed
+		}
+		return "", fmt.Errorf("checking %s: %w", bin, err)
+	}
+	return bin, nil
+}
+
+// Find searches for the required Java version, downloading it if necessary.
+// Returns the path to the java binary.
+func Find(version int) (string, error) {
+	bin, err := IsInstalled(version)
+	if err != nil {
+		return "", err
+	}
+	if bin != "" {
+		return bin, nil
+	}
+
+	// TODO: Download and extract JRE for the requested version.
+	// For now, return an error with instructions.
+	return "", fmt.Errorf(
+		"Java %d is not installed. Please wait for auto-download feature "+
+			"(planned) or manually place JRE in %s",
+		version,
+		must(config.JavaDir(version)),
+	)
+}
+
+// must panics on error — only used for error messages where the dir is already known valid.
+func must(s string, err error) string {
+	if err != nil {
+		panic(err)
+	}
+	return s
+}