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/projectView.js

@@ -74,6 +74,7 @@ export async function renderProjectView(app) {
 
     appEl.innerHTML = `
       <header class="app-header">
+        <button class="mobile-nav-btn" onclick="toggleMobileSidebar()" title="Menu">☰</button>
         <div class="logo">📝 SimpleNote</div>
         <div class="breadcrumb-nav">
           <span class="breadcrumb-link" onclick="window.app.navigate('projects')">Projects</span>
@@ -85,8 +86,10 @@ export async function renderProjectView(app) {
           <button class="btn btn-ghost" onclick="window.confirmDeleteProject('${project.id}')" title="Delete Project">🗑️</button>
         </div>
       </header>
+      <div class="sidebar-overlay" onclick="closeMobileSidebar()"></div>
       <div class="app-layout">
-        <aside class="sidebar project-sidebar">
+        <aside class="sidebar project-sidebar" id="sidebar">
+          <button class="sidebar-close-btn" onclick="closeMobileSidebar()">✕</button>
           <div class="sidebar-scroll">
             <div class="sidebar-section">
               <div class="section-header">
@@ -130,7 +133,7 @@ export async function renderProjectView(app) {
                 <span class="icon">🔍</span>
                 <input type="text" id="search-input" placeholder="Search documents..." value="${escapeHtml(searchQuery)}">
               </div>
-              <button class="btn btn-primary" onclick="window.showNewDocModal('${projectId}', '${selectedFolderId || ''}')">+ New Document</button>
+              <button class="btn btn-primary" onclick="window.showNewDocModal('${projectId}', '${selectedFolderId || ''}')">+ New</button>
             </div>
           </div>
           <div class="content-body">
@@ -155,6 +158,25 @@ export async function renderProjectView(app) {
     attachEventListeners();
   }
 
+  // Mobile sidebar functions
+  window.toggleMobileSidebar = function() {
+    const sidebar = document.getElementById('sidebar');
+    const overlay = document.querySelector('.sidebar-overlay');
+    if (sidebar) {
+      sidebar.classList.toggle('mobile-open');
+      if (overlay) overlay.classList.toggle('active');
+    }
+  };
+
+  window.closeMobileSidebar = function() {
+    const sidebar = document.getElementById('sidebar');
+    const overlay = document.querySelector('.sidebar-overlay');
+    if (sidebar) {
+      sidebar.classList.remove('mobile-open');
+      if (overlay) overlay.classList.remove('active');
+    }
+  };
+
   function attachEventListeners() {
     // Search
     const searchInput = document.getElementById('search-input');