Add XS pen size and make it the default

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

src/App.jsx

@@ -1321,7 +1321,7 @@ const App = () => {
   ); // 'stacked' | 'split' | 'annotate'
   const [drawTool, setDrawTool] = useState('pen');
   const [drawColor, setDrawColor] = useState('#0f172a');
-  const [drawSize, setDrawSize] = useState(0.012);
+  const [drawSize, setDrawSize] = useState(0.003);
   const [activeStudyTab, setActiveStudyTab] = useState(
     () => localStorage.getItem('activeStudyTab') || 'notes',
   );

src/pages/DrawCanvas.jsx

@@ -4,6 +4,7 @@ import { renderInkToCanvas } from '../utils/inkRender.js';
 
 const INK_COLORS = ['#0f172a', '#ef4444', '#3b82f6', '#16a34a', '#f59e0b', '#a855f7'];
 const INK_SIZES = [
+  { label: 'XS', value: 0.003 },
   { label: 'S', value: 0.006 },
   { label: 'M', value: 0.012 },
   { label: 'L', value: 0.025 },