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>

cmd/launcher/main.go

@@ -2,21 +2,53 @@ package main
 
 import (
 	"fmt"
+	"log"
 
 	"fyne.io/fyne/v2/app"
 	"fyne.io/fyne/v2/container"
 	"fyne.io/fyne/v2/widget"
+
+	"gitea.mrixs.me/Mrixs/MrixsCraft-launcher/internal/auth"
+	"gitea.mrixs.me/Mrixs/MrixsCraft-launcher/internal/config"
 )
 
-func main() {
-	fmt.Println("MrixsCraft Launcher starting...")
+// version is set via -ldflags at build time.
+var version = "dev"
 
+func main() {
+	log.Printf("MrixsCraft Launcher %s", version)
+
+	root, err := config.EnsureRoot()
+	if err != nil {
+		log.Fatalf("Failed to initialise data directory: %v", err)
+	}
+	log.Printf("Data directory: %s", root)
+
+	// Try to restore an existing session.
+	client, err := auth.NewFromConfig()
+	if err != nil {
+		log.Fatalf("Failed to create auth client: %v", err)
+	}
+
+	sess, err := client.EnsureValid()
+	if err != nil {
+		log.Printf("Session check failed: %v", err)
+	}
+
+	if sess != nil {
+		log.Printf("Logged in as %s", sess.Username)
+	} else {
+		log.Println("No valid session — login required")
+	}
+
+	// Bootstrap Fyne UI.
 	a := app.New()
-	w := a.NewWindow("MrixsCraft Launcher")
-	w.Resize(fyne.NewSize(800, 600))
+	w := a.NewWindow(fmt.Sprintf("MrixsCraft %s", version))
+	w.Resize(fyne.NewSize(900, 600))
+	w.CenterOnScreen()
 
 	w.SetContent(container.NewVBox(
-		widget.NewLabel("MrixsCraft Launcher"),
+		widget.NewLabel(fmt.Sprintf("MrixsCraft Launcher %s", version)),
 	))
 
 	w.ShowAndRun()