Add Tauri launcher with Svelte UI

- Accounts: panel login/sign-up, Microsoft device-code sign-in, offline
  play; tokens encrypted with a key kept in the OS credential store
- Home: instance rail, live server status, news feed, social links,
  Play button with download progress, speed and ETA
- Full settings: accounts, memory/resolution/after-launch behaviour,
  Java and GC presets, Minecraft keybind editor applied to options.txt,
  theme overrides, storage manager, self-update via GitHub releases
- Game console, crash dialog with hints, per-instance overrides
- Live admin branding (colours, fonts, backgrounds, custom CSS)
- Online install tests for vanilla/Fabric/Quilt/Forge/NeoForge (CI)

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

launcher/src/main.ts

@@ -0,0 +1,15 @@
+import '@fontsource-variable/outfit';
+import '@fontsource-variable/inter';
+import '@fontsource-variable/space-grotesk';
+import '@fontsource-variable/plus-jakarta-sans';
+import '@fontsource-variable/jetbrains-mono';
+import './app.css';
+import { mount } from 'svelte';
+import App from './App.svelte';
+
+// The launcher is a desktop app: no browser context menu or text-drag.
+document.addEventListener('contextmenu', (e) => {
+  if (!(e.target as HTMLElement).closest('input, textarea, .selectable')) e.preventDefault();
+});
+
+mount(App, { target: document.getElementById('app')! });