feat: Matcha! social integration — friends, chat, DMs, avatars, presence

Add full Matcha social panel (butter.lat API) as a right-side slide-out:

Backend (matchaService.js):
- HTTP client for auth, friends, messages, unread, avatar, heartbeat APIs
- WebSocket with auto-reconnect (exponential backoff, no hard cap)
- Token management via config, presence heartbeat every 30s
- WS error message type handling, game running presence

Renderer (matcha.js):
- State machine UI: intro → login/register → app (friends/chat/DMs/profile)
- Two-phase registration with master key display and verification
- Friends list with presence dots, collapsible requests, 12s polling
- Global chat + DM with optimistic rendering, cursor pagination
- Scroll position preserved on load-more, separate loading flags
- Clickable URLs in messages (linkify with proper escaping)
- User profile popup with avatar upload/delete
- Unread badges (messages + friend requests) on nav icon
- Escape key closes panel/overlay, try/catch on auth flows

IPC bridge (preload.js + main.js):
- 21 IPC invoke methods + 8 WS event listeners
- Avatar upload via file picker dialog in main process
- Game launch sets in_game heartbeat state

CSS (style.css):
- ~1500 lines: panel, auth screens, friends, chat, profile, toast
- Responsive panel width, improved contrast, no overflow clipping
- Loading states, disabled states, pulse animations

Credits: Powered by Butter Launcher & Matcha! (butterlauncher.tech)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

GUI/js/script.js

@@ -5,6 +5,7 @@ import { initModsManager } from './mods.js';
 import './players.js';
 import './settings.js';
 import './logs.js';
+import { initMatcha } from './matcha.js';
 
 let i18nInitialized = false;
 (async () => {
@@ -15,10 +16,12 @@ let i18nInitialized = false;
   if (document.readyState === 'complete' || document.readyState === 'interactive') {
     updateLanguageSelector();
     initModsManager();
+    initMatcha();
   } else {
     document.addEventListener('DOMContentLoaded', () => {
       updateLanguageSelector();
       initModsManager();
+      initMatcha();
     });
   }
 })();