feat: add auth, real-time collaboration, sharing, font control, and UI fixes

Phase 1 - Bug fixes:
- Fix color labels not showing on active line in format preview
- Replace eye emoji with SVG icon showing clear preview/raw state
- Replace // button with comment icon + better tooltip
- Fix ThemePicker accent colors when using system theme

Phase 2 - Font:
- Load JetBrains Mono via Google Fonts with offline fallback
- Add font size control (A-/A+) with keyboard shortcuts
- Persist font size preference in localStorage

Phase 3 - Auth:
- Supabase-based email/password authentication
- Device session management with configurable password renewal TTL
- AuthModal, UserMenu, SecuritySettings components

Phase 4 - Cloud sync:
- Document metadata sync to Supabase PostgreSQL
- Legacy localStorage migration on first login
- IndexedDB persistence via y-indexeddb

Phase 5 - Real-time collaboration:
- Y.js CRDT integration with CodeMirror 6
- Hocuspocus WebSocket server with JWT auth
- Collaborative cursor awareness
- CollabIndicator component

Phase 6 - Sharing:
- Share links with view/edit permissions
- ShareDialog component with copy-to-clipboard
- Minimal client-side router for /s/{token} URLs

Infrastructure:
- Docker Compose with PostgreSQL, GoTrue, PostgREST, Hocuspocus
- Nginx reverse proxy for all backend services
- SQL migrations with RLS policies
- Production-ready Dockerfile with build args

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

calcpad-web/index.html

@@ -10,6 +10,9 @@
     <meta name="apple-mobile-web-app-status-bar-style" content="default" />
     <meta name="apple-mobile-web-app-title" content="CalcPad" />
     <link rel="apple-touch-icon" href="/icons/icon-192.svg" />
+    <link rel="preconnect" href="https://fonts.googleapis.com" />
+    <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin />
+    <link href="https://fonts.googleapis.com/css2?family=JetBrains+Mono:wght@400;500;600;700&display=swap" rel="stylesheet" />
     <title>CalcText</title>
     <script>
       // Apply theme before React mounts to prevent FOUC
@@ -20,6 +23,9 @@
           t = window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light';
         }
         document.documentElement.setAttribute('data-theme', t);
+        // Apply font size before React mounts
+        var fs = localStorage.getItem('calctext-fontsize');
+        if (fs) document.documentElement.style.setProperty('--editor-font-size', fs + 'px');
       })();
     </script>
   </head>