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/theme/theme.go

@@ -1,2 +1,92 @@
-// package theme defines the custom Fyne theme (Minecraft-style colors, fonts).
+// package theme defines a custom Fyne theme with Minecraft-inspired colors.
 package theme
+
+import (
+	"image/color"
+
+	"fyne.io/fyne/v2"
+	"fyne.io/fyne/v2/theme"
+)
+
+// MinecraftTheme is a dark theme inspired by Minecraft's UI palette.
+type MinecraftTheme struct{}
+
+var _ fyne.Theme = (*MinecraftTheme)(nil)
+
+// Color returns the named color from the Minecraft palette.
+func (t *MinecraftTheme) Color(name fyne.ThemeColorName, variant fyne.ThemeVariant) color.Color {
+	switch name {
+	case theme.ColorNameBackground:
+		return color.RGBA{R: 30, G: 30, B: 30, A: 255} // dark stone
+	case theme.ColorNameButton:
+		return color.RGBA{R: 100, G: 100, B: 100, A: 255} // stone gray
+	case theme.ColorNameDisabled:
+		return color.RGBA{R: 80, G: 80, B: 80, A: 255}
+	case theme.ColorNameDisabledButton:
+		return color.RGBA{R: 60, G: 60, B: 60, A: 255}
+	case theme.ColorNameError:
+		return color.RGBA{R: 200, G: 50, B: 50, A: 255}
+	case theme.ColorNameFocus:
+		return color.RGBA{R: 76, G: 175, B: 80, A: 255} // green accent
+	case theme.ColorNameForeground:
+		return color.RGBA{R: 220, G: 220, B: 220, A: 255} // light gray
+	case theme.ColorNameHover:
+		return color.RGBA{R: 76, G: 175, B: 80, A: 255} // green
+	case theme.ColorNameInputBackground:
+		return color.RGBA{R: 50, G: 50, B: 50, A: 255}
+	case theme.ColorNameInputBorder:
+		return color.RGBA{R: 80, G: 80, B: 80, A: 255}
+	case theme.ColorNameMenuBackground:
+		return color.RGBA{R: 40, G: 40, B: 40, A: 255}
+	case theme.ColorNameOverlayBackground:
+		return color.RGBA{R: 0, G: 0, B: 0, A: 180}
+	case theme.ColorNamePlaceHolder:
+		return color.RGBA{R: 150, G: 150, B: 150, A: 255}
+	case theme.ColorNamePressed:
+		return color.RGBA{R: 56, G: 142, B: 60, A: 255} // dark green
+	case theme.ColorNamePrimary:
+		return color.RGBA{R: 76, G: 175, B: 80, A: 255} // green
+	case theme.ColorNameScrollBar:
+		return color.RGBA{R: 80, G: 80, B: 80, A: 200}
+	case theme.ColorNameSeparator:
+		return color.RGBA{R: 60, G: 60, B: 60, A: 255}
+	case theme.ColorNameShadow:
+		return color.RGBA{R: 0, G: 0, B: 0, A: 128}
+	default:
+		return theme.DefaultTheme().Color(name, variant)
+	}
+}
+
+// Font returns the default font (custom TTF can be added later).
+func (t *MinecraftTheme) Font(style fyne.TextStyle) fyne.Resource {
+	return theme.DefaultTheme().Font(style)
+}
+
+// Icon returns the named icon resource.
+func (t *MinecraftTheme) Icon(name fyne.ThemeIconName) fyne.Resource {
+	return theme.DefaultTheme().Icon(name)
+}
+
+// Size returns the named size value.
+func (t *MinecraftTheme) Size(name fyne.ThemeSizeName) float32 {
+	switch name {
+	case theme.SizeNameCaptionText:
+		return 11
+	case theme.SizeNameInlineIcon:
+		return 20
+	case theme.SizeNamePadding:
+		return 4
+	case theme.SizeNameScrollBar:
+		return 12
+	case theme.SizeNameScrollBarSmall:
+		return 4
+	case theme.SizeNameText:
+		return 14
+	case theme.SizeNameHeadingText:
+		return 24
+	case theme.SizeNameSubHeadingText:
+		return 18
+	default:
+		return theme.DefaultTheme().Size(name)
+	}
+}