feat: admin panel login, layout, and styles

Add the admin CRM panel foundation including:
- Login/logout authentication flow with CSRF protection
- Sidebar layout with navigation for all CRM modules
- Comprehensive admin.css (1680+ lines) with components for
  cards, tables, badges, forms, buttons, alerts, modals,
  pagination, search, responsive breakpoints, and print styles
- admin.js with sidebar toggle, confirm dialogs, auto-dismiss
  alerts, and table search filtering

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

assets/js/admin.js

@@ -0,0 +1,55 @@
+document.addEventListener('DOMContentLoaded', function() {
+    // Sidebar toggle
+    const sidebar = document.getElementById('sidebar');
+    const sidebarToggle = document.getElementById('sidebarToggle');
+    const sidebarOverlay = document.getElementById('sidebarOverlay');
+
+    if (sidebarToggle) {
+        sidebarToggle.addEventListener('click', function() {
+            sidebar.classList.toggle('sidebar--open');
+            sidebarOverlay.classList.toggle('active');
+            document.body.classList.toggle('sidebar-open');
+        });
+    }
+
+    if (sidebarOverlay) {
+        sidebarOverlay.addEventListener('click', function() {
+            sidebar.classList.remove('sidebar--open');
+            sidebarOverlay.classList.remove('active');
+            document.body.classList.remove('sidebar-open');
+        });
+    }
+
+    // Confirm delete actions
+    document.querySelectorAll('[data-confirm]').forEach(function(el) {
+        el.addEventListener('click', function(e) {
+            if (!confirm(this.dataset.confirm)) {
+                e.preventDefault();
+            }
+        });
+    });
+
+    // Auto-dismiss alerts after 5 seconds
+    document.querySelectorAll('.alert--dismissible').forEach(function(alert) {
+        setTimeout(function() {
+            alert.style.opacity = '0';
+            alert.style.transform = 'translateY(-10px)';
+            setTimeout(function() { alert.remove(); }, 300);
+        }, 5000);
+    });
+
+    // Table search filter
+    var searchInput = document.getElementById('tableSearch');
+    if (searchInput) {
+        searchInput.addEventListener('input', function() {
+            var query = this.value.toLowerCase();
+            var table = document.querySelector('.admin-table');
+            if (!table) return;
+            var rows = table.querySelectorAll('tbody tr');
+            rows.forEach(function(row) {
+                var text = row.textContent.toLowerCase();
+                row.style.display = text.includes(query) ? '' : 'none';
+            });
+        });
+    }
+});