Add admin panel web UI (Svelte 5)

Dashboard with launch stats, instance manager with Modrinth/CurseForge
search and zip import, player and group management, launcher design
editor with live preview, and sign-in settings.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011ARcGWxLx21FwXJ3yfGriS

panel/web/svelte.config.js

@@ -0,0 +1,3 @@
+import { vitePreprocess } from '@sveltejs/vite-plugin-svelte';
+
+export default { preprocess: vitePreprocess() };