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/ui.go

@@ -1,2 +1,55 @@
-// package ui contains Fyne GUI code (main window, theme, navigation).
+// package ui contains the Fyne GUI bootstrap and wiring.
 package ui
+
+import (
+	"fyne.io/fyne/v2"
+	"fyne.io/fyne/v2/app"
+
+	"gitea.mrixs.me/Mrixs/MrixsCraft-launcher/internal/auth"
+	"gitea.mrixs.me/Mrixs/MrixsCraft-launcher/internal/config"
+	"gitea.mrixs.me/Mrixs/MrixsCraft-launcher/internal/ui/screens"
+	"gitea.mrixs.me/Mrixs/MrixsCraft-launcher/internal/ui/theme"
+)
+
+// Launch initialises the Fyne application with the Minecraft theme and shows the main window.
+func Launch(client *auth.Client, session *auth.Session, settings config.Settings) {
+	a := app.New()
+	a.Settings().SetTheme(&theme.MinecraftTheme{})
+
+	w := a.NewWindow("MrixsCraft")
+	w.Resize(fyne.NewSize(float32(settings.Width), float32(settings.Height)))
+	w.CenterOnScreen()
+
+	// TODO: fetch from /api/servers.json
+	serverList := []screens.Modpack{
+		{Slug: "hitech", Name: "HiTech 1.21"},
+		{Slug: "vanilla", Name: "Vanilla 1.20"},
+	}
+
+	onPlay := func() {
+		// TODO: launch selected modpack
+	}
+
+	onSettings := func() {
+		screens.SettingsScreen(w, func(memoryMB int, extraArgs string) {
+			settings.MemoryMB = memoryMB
+			settings.ExtraArgs = extraArgs
+			_ = config.Save(settings)
+		})
+	}
+
+	content := screens.MainScreen(w, client, session, serverList, onPlay, onSettings)
+	w.SetContent(content)
+
+	// If no session, show login modal after the window is rendered.
+	if session == nil {
+		w.Show()
+		screens.LoginScreen(w, client, func(sess *auth.Session) {
+			// Refresh UI with logged-in state.
+			content := screens.MainScreen(w, client, sess, serverList, onPlay, onSettings)
+			w.SetContent(content)
+		})
+	}
+
+	w.ShowAndRun()
+}