feat: Dashboard placeholder avec preview Score Sérénité

Permet aux parents de visualiser une démo du Score Sérénité dès leur
première connexion, avant même que les données réelles soient disponibles.
Les autres rôles (enseignant, élève, admin) ont également leur dashboard
adapté avec des sections placeholder.

La landing page redirige automatiquement vers /dashboard si l'utilisateur
est déjà authentifié, offrant un accès direct au tableau de bord.

frontend/tests/unit/lib/features/dashboard/serenity-score.test.ts

@@ -0,0 +1,106 @@
+import { describe, it, expect } from 'vitest';
+import {
+	calculateSerenityScore,
+	getSerenityEmoji,
+	getSerenityLabel,
+	isValidScore,
+	getTrendIcon
+} from '$lib/features/dashboard/serenity-score';
+
+describe('serenity-score utilities', () => {
+	describe('calculateSerenityScore', () => {
+		it('should calculate score correctly with all 100s', () => {
+			const score = calculateSerenityScore({ notes: 100, absences: 100, devoirs: 100 });
+			expect(score).toBe(100);
+		});
+
+		it('should calculate score correctly with all 0s', () => {
+			const score = calculateSerenityScore({ notes: 0, absences: 0, devoirs: 0 });
+			expect(score).toBe(0);
+		});
+
+		it('should calculate score with weighted components', () => {
+			// Notes 90 × 0.4 = 36
+			// Absences 95 × 0.3 = 28.5
+			// Devoirs 70 × 0.3 = 21
+			// Total = 85.5, rounded to 86
+			const score = calculateSerenityScore({ notes: 90, absences: 95, devoirs: 70 });
+			expect(score).toBe(86);
+		});
+
+		it('should round the score to nearest integer', () => {
+			const score = calculateSerenityScore({ notes: 50, absences: 50, devoirs: 50 });
+			expect(score).toBe(50);
+			expect(Number.isInteger(score)).toBe(true);
+		});
+	});
+
+	describe('getSerenityEmoji', () => {
+		it('should return green emoji for scores >= 70', () => {
+			expect(getSerenityEmoji(70)).toBe('💚');
+			expect(getSerenityEmoji(85)).toBe('💚');
+			expect(getSerenityEmoji(100)).toBe('💚');
+		});
+
+		it('should return yellow emoji for scores 40-69', () => {
+			expect(getSerenityEmoji(40)).toBe('🟡');
+			expect(getSerenityEmoji(55)).toBe('🟡');
+			expect(getSerenityEmoji(69)).toBe('🟡');
+		});
+
+		it('should return red emoji for scores < 40', () => {
+			expect(getSerenityEmoji(0)).toBe('🔴');
+			expect(getSerenityEmoji(25)).toBe('🔴');
+			expect(getSerenityEmoji(39)).toBe('🔴');
+		});
+	});
+
+	describe('getSerenityLabel', () => {
+		it('should return Excellent for scores >= 70', () => {
+			expect(getSerenityLabel(70)).toBe('Excellent');
+			expect(getSerenityLabel(100)).toBe('Excellent');
+		});
+
+		it('should return A surveiller for scores 40-69', () => {
+			expect(getSerenityLabel(40)).toBe('A surveiller');
+			expect(getSerenityLabel(69)).toBe('A surveiller');
+		});
+
+		it('should return Attention requise for scores < 40', () => {
+			expect(getSerenityLabel(0)).toBe('Attention requise');
+			expect(getSerenityLabel(39)).toBe('Attention requise');
+		});
+	});
+
+	describe('isValidScore', () => {
+		it('should return true for valid scores', () => {
+			expect(isValidScore(0)).toBe(true);
+			expect(isValidScore(50)).toBe(true);
+			expect(isValidScore(100)).toBe(true);
+		});
+
+		it('should return false for scores below 0', () => {
+			expect(isValidScore(-1)).toBe(false);
+			expect(isValidScore(-100)).toBe(false);
+		});
+
+		it('should return false for scores above 100', () => {
+			expect(isValidScore(101)).toBe(false);
+			expect(isValidScore(200)).toBe(false);
+		});
+	});
+
+	describe('getTrendIcon', () => {
+		it('should return up arrow for up trend', () => {
+			expect(getTrendIcon('up')).toBe('↑');
+		});
+
+		it('should return down arrow for down trend', () => {
+			expect(getTrendIcon('down')).toBe('↓');
+		});
+
+		it('should return right arrow for stable trend', () => {
+			expect(getTrendIcon('stable')).toBe('→');
+		});
+	});
+});