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/components/components.go

@@ -1,2 +1,94 @@
-// package components provides reusable Fyne widgets (avatar, progress bar).
+// package components provides reusable Fyne widgets for the launcher UI.
 package components
+
+import (
+	"image/color"
+
+	"fyne.io/fyne/v2"
+	"fyne.io/fyne/v2/canvas"
+	"fyne.io/fyne/v2/container"
+	"fyne.io/fyne/v2/widget"
+)
+
+// ServerCard displays a single modpack entry in the sidebar.
+func ServerCard(name string, selected bool, onTap func()) fyne.CanvasObject {
+	bg := canvas.NewRectangle(color.Transparent)
+	bg.SetMinSize(fyne.NewSize(180, 48))
+
+	label := widget.NewLabel(name)
+	label.TextStyle.Bold = selected
+
+	content := container.NewBorder(nil, nil,
+		nil, nil,
+		label,
+	)
+
+	card := container.NewStack(bg, content)
+
+	btn := widget.NewButton("", onTap)
+	btn.Importance = widget.LowImportance
+
+	return container.NewStack(card, btn)
+}
+
+// ProgressBar wraps widget.ProgressBar with show/hide helpers.
+type ProgressBar struct {
+	*widget.ProgressBar
+	hidden bool
+}
+
+// NewProgressBar creates a new progress bar initialized to 0 and hidden.
+func NewProgressBar() *ProgressBar {
+	pb := widget.NewProgressBar()
+	pb.Hide()
+	return &ProgressBar{
+		ProgressBar: pb,
+		hidden:      true,
+	}
+}
+
+// SetProgress sets the value [0…1] and shows the bar.
+func (pb *ProgressBar) SetProgress(v float64) {
+	pb.SetValue(v)
+	if pb.hidden {
+		pb.Show()
+		pb.hidden = false
+	}
+}
+
+// Done hides the progress bar and resets it.
+func (pb *ProgressBar) Done() {
+	pb.SetValue(0)
+	pb.Hide()
+	pb.hidden = true
+}
+
+// PlayButton creates the accent "PLAY" button.
+func PlayButton(onTap func()) *widget.Button {
+	btn := widget.NewButton("▶  PLAY", onTap)
+	btn.Importance = widget.HighImportance
+	return btn
+}
+
+// SettingsButton creates the small settings (gear) button.
+func SettingsButton(onTap func()) *widget.Button {
+	btn := widget.NewButton("⚙", onTap)
+	btn.Importance = widget.LowImportance
+	return btn
+}
+
+// LogoutButton creates the logout button.
+func LogoutButton(onTap func()) *widget.Button {
+	btn := widget.NewButton("Выйти", onTap)
+	btn.Importance = widget.LowImportance
+	return btn
+}
+
+// AvatarImage returns a placeholder avatar (64×64 colored square).
+// TODO: render 8×64 face from skin PNG.
+func AvatarImage() *canvas.Image {
+	img := canvas.NewImageFromResource(nil)
+	img.SetMinSize(fyne.NewSize(64, 64))
+	img.FillMode = canvas.ImageFillContain
+	return img
+}