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>

internal/ui/screens/screens.go

@@ -1,2 +1,193 @@
 // package screens implements application screens (login, main menu, settings).
 package screens
+
+import (
+	"fmt"
+
+	"fyne.io/fyne/v2"
+	"fyne.io/fyne/v2/container"
+	"fyne.io/fyne/v2/dialog"
+	"fyne.io/fyne/v2/layout"
+	"fyne.io/fyne/v2/widget"
+
+	"gitea.mrixs.me/Mrixs/MrixsCraft-launcher/internal/auth"
+	"gitea.mrixs.me/Mrixs/MrixsCraft-launcher/internal/ui/components"
+)
+
+// Modpack represents a single server/modpack entry from the backend.
+type Modpack struct {
+	Slug string
+	Name string
+}
+
+// MainScreen builds the primary launcher window content.
+func MainScreen(
+	w fyne.Window,
+	client *auth.Client,
+	session *auth.Session,
+	serverList []Modpack,
+	onPlay func(),
+	onSettings func(),
+) fyne.CanvasObject {
+
+	// ── Sidebar (Left) ──────────────────────────────────────
+	serverLabels := make([]*widget.Label, len(serverList))
+	serverContainer := container.NewVBox()
+
+	for i, sp := range serverList {
+		label := widget.NewLabel(sp.Name)
+		serverLabels[i] = label
+		index := i
+		card := components.ServerCard(sp.Name, index == 0, func() {
+			for j, l := range serverLabels {
+				l.TextStyle.Bold = (j == index)
+				l.Refresh()
+			}
+		})
+		serverContainer.Add(card)
+	}
+
+	sidebar := container.NewVScroll(serverContainer)
+
+	// ── Center (background + info) ──────────────────────────
+	bg := widget.NewRichTextFromMarkdown(
+		"# Welcome to MrixsCraft\n\n" +
+			"Select a server from the left panel and press **PLAY**.\n\n" +
+			"---\n\n" +
+			"*Version: dev*",
+	)
+	center := container.NewMax(widget.NewCard("", "", bg))
+
+	// ── Bottom bar ──────────────────────────────────────────
+	avatar := components.AvatarImage()
+
+	displayName := "Not logged in"
+	if session != nil && session.Username != "" {
+		displayName = session.Username
+	}
+	usernameLabel := widget.NewLabel(displayName)
+
+	logoutBtn := components.LogoutButton(func() {
+		dialog.ShowConfirm(
+			"Logout",
+			"Are you sure you want to log out?",
+			func(confirmed bool) {
+				if confirmed {
+					_ = auth.Delete()
+					w.Close()
+				}
+			},
+			w,
+		)
+	})
+
+	bottomLeft := container.NewHBox(
+		avatar,
+		usernameLabel,
+		layout.NewSpacer(),
+		logoutBtn,
+	)
+
+	progress := components.NewProgressBar()
+
+	playBtn := components.PlayButton(onPlay)
+	settingsBtn := components.SettingsButton(onSettings)
+
+	bottomRight := container.NewHBox(
+		settingsBtn,
+		widget.NewSeparator(),
+		playBtn,
+	)
+
+	bottomCenter := container.NewMax(progress)
+
+	bottom := container.NewBorder(
+		nil, nil,
+		bottomLeft,
+		bottomRight,
+		bottomCenter,
+	)
+
+	// ── Assemble ────────────────────────────────────────────
+	return container.NewBorder(
+		nil,
+		bottom,
+		sidebar,
+		nil,
+		center,
+	)
+}
+
+// LoginScreen builds a modal login form.
+func LoginScreen(w fyne.Window, client *auth.Client, onLogin func(*auth.Session)) {
+	username := widget.NewEntry()
+	username.SetPlaceHolder("Username or email")
+
+	password := widget.NewPasswordEntry()
+	password.SetPlaceHolder("Password")
+
+	form := widget.NewForm(
+		widget.NewFormItem("Username", username),
+		widget.NewFormItem("Password", password),
+	)
+
+	dialog.ShowCustomConfirm(
+		"Login — MrixsCraft",
+		"Login",
+		"Cancel",
+		form,
+		func(confirmed bool) {
+			if !confirmed {
+				return
+			}
+			sess, err := client.Authenticate(username.Text, password.Text)
+			if err != nil {
+				dialog.ShowError(err, w)
+				return
+			}
+			if err := sess.Save(); err != nil {
+				dialog.ShowError(err, w)
+				return
+			}
+			onLogin(sess)
+		},
+		w,
+	)
+}
+
+// SettingsScreen builds the settings modal.
+func SettingsScreen(w fyne.Window, onSave func(memoryMB int, extraArgs string)) {
+	memorySlider := widget.NewSlider(1024, 16384)
+	memorySlider.Step = 512
+	memorySlider.Value = 4096
+
+	memLabel := widget.NewLabel("4096 MB")
+	memorySlider.OnChanged = func(v float64) {
+		memLabel.SetText(fmt.Sprintf("%.0f MB", v))
+	}
+
+	extraArgs := widget.NewMultiLineEntry()
+	extraArgs.SetPlaceHolder("e.g. -XX:+UseG1GC -XX:MaxGCPauseMillis=50")
+
+	content := container.NewVBox(
+		widget.NewLabelWithStyle("Memory (RAM)", fyne.TextAlignLeading, fyne.TextStyle{Bold: true}),
+		memLabel,
+		memorySlider,
+		widget.NewSeparator(),
+		widget.NewLabelWithStyle("Extra JVM arguments", fyne.TextAlignLeading, fyne.TextStyle{}),
+		extraArgs,
+	)
+
+	dialog.ShowCustomConfirm(
+		"Settings",
+		"Save",
+		"Cancel",
+		content,
+		func(confirmed bool) {
+			if confirmed {
+				onSave(int(memorySlider.Value), extraArgs.Text)
+			}
+		},
+		w,
+	)
+}