feat: add UI layer (theme, components, screens, main window)

- theme: MinecraftTheme — dark palette with green accents
- components: ServerCard, ProgressBar, PlayButton, SettingsButton, LogoutButton, AvatarImage
- screens: MainScreen (BorderLayout: sidebar + center + bottom bar), LoginScreen (modal form), SettingsScreen (RAM slider + JVM args)
- ui.Launch: wires theme, session, settings into Fyne app
- main.go: delegates to ui.Launch, loads config
- fix: nil-session guard in MainScreen username display

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

cmd/launcher/main.go

@@ -1,15 +1,11 @@
 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"
+	"gitea.mrixs.me/Mrixs/MrixsCraft-launcher/internal/ui"
 )
 
 // version is set via -ldflags at build time.
@@ -24,7 +20,11 @@ func main() {
 	}
 	log.Printf("Data directory: %s", root)
 
-	// Try to restore an existing session.
+	settings, err := config.Load()
+	if err != nil {
+		log.Fatalf("Failed to load settings: %v", err)
+	}
+
 	client, err := auth.NewFromConfig()
 	if err != nil {
 		log.Fatalf("Failed to create auth client: %v", err)
@@ -41,15 +41,5 @@ func main() {
 		log.Println("No valid session — login required")
 	}
 
-	// Bootstrap Fyne UI.
-	a := app.New()
-	w := a.NewWindow(fmt.Sprintf("MrixsCraft %s", version))
-	w.Resize(fyne.NewSize(900, 600))
-	w.CenterOnScreen()
-
-	w.SetContent(container.NewVBox(
-		widget.NewLabel(fmt.Sprintf("MrixsCraft Launcher %s", version)),
-	))
-
-	w.ShowAndRun()
+	ui.Launch(client, sess, settings)
 }