refactor(ui): unify frontend around compact, desktop-first design

Extract shared utilities (color functions, icon registry), replace hero
banners with compact PageHeader, add TrimesterSelector/ConfirmDialog/
Breadcrumb components, consolidate off-palette colors to design tokens,
convert AssessmentListView to table layout, compress ResultsView stats
into horizontal bar, and inline ClassFormView as a modal in ClassListView.

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

frontend/tailwind.config.js

@@ -22,20 +22,46 @@ export default {
         success: {
           50: '#f0fdf4',
           100: '#dcfce7',
+          200: '#bbf7d0',
+          300: '#86efac',
+          400: '#4ade80',
           500: '#22c55e',
           600: '#16a34a',
+          700: '#15803d',
+          800: '#166534',
         },
         warning: {
           50: '#fffbeb',
           100: '#fef3c7',
+          200: '#fde68a',
+          300: '#fcd34d',
+          400: '#fbbf24',
           500: '#f59e0b',
           600: '#d97706',
+          700: '#b45309',
+          800: '#92400e',
         },
         danger: {
           50: '#fef2f2',
           100: '#fee2e2',
+          200: '#fecaca',
+          300: '#fca5a5',
+          400: '#f87171',
           500: '#ef4444',
           600: '#dc2626',
+          700: '#b91c1c',
+          800: '#991b1b',
+        },
+        accent: {
+          50: '#faf5ff',
+          100: '#f3e8ff',
+          200: '#e9d5ff',
+          300: '#d8b4fe',
+          400: '#c084fc',
+          500: '#a855f7',
+          600: '#9333ea',
+          700: '#7e22ce',
+          800: '#6b21a8',
         }
       }
     },