feat: improve mobile responsiveness

- Add mobile navigation drawer with hamburger menu
- Sidebar slides in as overlay on mobile (<768px)
- Close button inside sidebar for mobile
- Ensure touch targets are at least 44px on mobile
- Make modals full-screen on mobile
- Editor toolbar scrolls horizontally on mobile
- Improve spacing and typography for small screens
- Keep dark theme consistent across breakpoints
- Projects page cards stack vertically on mobile
- Document cards full-width on mobile

public/js/views/projects.js

@@ -19,7 +19,7 @@ export async function renderProjects(app) {
       <header class="app-header">
         <div class="logo">📝 SimpleNote</div>
         <div class="header-actions">
-          <button class="btn btn-primary" onclick="window.showNewProjectModal()">+ New Project</button>
+          <button class="btn btn-primary" onclick="window.showNewProjectModal()">+ New</button>
         </div>
       </header>
       <div class="projects-page">