feat: Bloquer la création de devoirs non conformes en mode hard

Les établissements utilisant le mode "Hard" des règles de devoirs
empêchent désormais les enseignants de créer des devoirs hors règles.
Contrairement au mode "Soft" (avertissement avec possibilité de passer
outre), le mode "Hard" est un blocage strict : même acknowledgeWarning
ne permet pas de contourner.

L'API retourne 422 (au lieu de 409 pour le soft) avec des dates
conformes suggérées calculées via le calendrier scolaire (weekends,
fériés, vacances exclus). Le frontend affiche un modal de blocage
avec les raisons, des dates cliquables, et une validation client
inline qui empêche la soumission de dates non conformes.

frontend/src/routes/dashboard/teacher/homework/+page.svelte

@@ -4,6 +4,7 @@
 	import { getApiBaseUrl } from '$lib/api/config';
 	import { authenticatedFetch, getAuthenticatedUserId } from '$lib/auth';
 	import Pagination from '$lib/components/molecules/Pagination/Pagination.svelte';
+	import RuleBlockedModal from '$lib/components/molecules/RuleBlockedModal/RuleBlockedModal.svelte';
 	import SearchInput from '$lib/components/molecules/SearchInput/SearchInput.svelte';
 	import { untrack } from 'svelte';
 
@@ -94,11 +95,19 @@
 	let duplicateValidationResults = $state<Array<{ classId: string; valid: boolean; error: string | null }>>([]);
 	let duplicateWarnings = $state<Array<{ classId: string; warning: string }>>([]);
 
-	// Rule warning modal
+	// Rule warning modal (soft mode)
 	let showRuleWarningModal = $state(false);
 	let ruleWarnings = $state<RuleWarning[]>([]);
 	let ruleConformMinDate = $state('');
 
+	// Rule blocked modal (hard mode)
+	let showRuleBlockedModal = $state(false);
+	let ruleBlockedWarnings = $state<RuleWarning[]>([]);
+	let ruleBlockedSuggestedDates = $state<string[]>([]);
+
+	// Inline date validation for hard mode
+	let dueDateError = $state<string | null>(null);
+
 	// Class filter
 	let filterClassId = $state(page.url.searchParams.get('classId') ?? '');
 
@@ -300,6 +309,7 @@
 		newDescription = '';
 		newDueDate = '';
 		ruleConformMinDate = '';
+		dueDateError = null;
 	}
 
 	function closeCreateModal() {
@@ -308,6 +318,8 @@
 
 	async function handleCreate(acknowledgeWarning = false) {
 		if (!newClassId || !newSubjectId || !newTitle.trim() || !newDueDate) return;
+		dueDateError = validateDueDateLocally(newDueDate);
+		if (dueDateError) return;
 
 		try {
 			isSubmitting = true;
@@ -326,6 +338,17 @@
 				}),
 			});
 
+			if (response.status === 422) {
+				const data = await response.json().catch(() => null);
+				if (data?.type === 'homework_rules_blocked' && Array.isArray(data.warnings)) {
+					ruleBlockedWarnings = data.warnings;
+					ruleBlockedSuggestedDates = Array.isArray(data.suggestedDates) ? data.suggestedDates : [];
+					showCreateModal = false;
+					showRuleBlockedModal = true;
+					return;
+				}
+			}
+
 			if (response.status === 409) {
 				const data = await response.json().catch(() => null);
 				if (data?.type === 'homework_rules_warning' && Array.isArray(data.warnings)) {
@@ -349,6 +372,9 @@
 			closeCreateModal();
 			showRuleWarningModal = false;
 			ruleWarnings = [];
+			showRuleBlockedModal = false;
+			ruleBlockedWarnings = [];
+			ruleBlockedSuggestedDates = [];
 			await loadHomeworks();
 		} catch (e) {
 			error = e instanceof Error ? e.message : 'Erreur lors de la création';
@@ -399,6 +425,44 @@
 		newDueDate = ruleConformMinDate;
 	}
 
+	function validateDueDateLocally(dateStr: string): string | null {
+		if (!dateStr) return null;
+		const date = new Date(dateStr + 'T00:00:00');
+		const day = date.getDay();
+		if (day === 0 || day === 6) {
+			return 'Les devoirs ne peuvent pas être fixés un weekend.';
+		}
+		if (ruleConformMinDate && dateStr < ruleConformMinDate) {
+			return 'Cette date ne respecte pas les règles de l\'établissement. Choisissez une date ultérieure.';
+		}
+		return null;
+	}
+
+	function handleDueDateChange(dateStr: string) {
+		newDueDate = dateStr;
+		dueDateError = validateDueDateLocally(dateStr);
+	}
+
+	function handleBlockedSelectDate(date: string) {
+		showRuleBlockedModal = false;
+		ruleBlockedWarnings = [];
+		ruleBlockedSuggestedDates = [];
+		ruleConformMinDate = date;
+		newDueDate = date;
+		showCreateModal = true;
+	}
+
+	function handleBlockedClose() {
+		const firstSuggested = ruleBlockedSuggestedDates[0];
+		const conformDate = firstSuggested ?? computeConformMinDate(ruleBlockedWarnings);
+		showRuleBlockedModal = false;
+		ruleBlockedWarnings = [];
+		ruleBlockedSuggestedDates = [];
+		ruleConformMinDate = conformDate;
+		newDueDate = conformDate;
+		showCreateModal = true;
+	}
+
 	// --- Edit ---
 	function openEditModal(hw: Homework) {
 		editHomework = hw;
@@ -779,8 +843,17 @@
 
 				<div class="form-group">
 					<label for="hw-due-date">Date d'échéance *</label>
-					<input type="date" id="hw-due-date" bind:value={newDueDate} required min={ruleConformMinDate || minDueDate} />
-					{#if ruleConformMinDate}
+					<input
+						type="date"
+						id="hw-due-date"
+						value={newDueDate}
+						oninput={(e) => handleDueDateChange((e.target as HTMLInputElement).value)}
+						required
+						min={ruleConformMinDate || minDueDate}
+					/>
+					{#if dueDateError}
+						<small class="form-hint form-hint-error">{dueDateError}</small>
+					{:else if ruleConformMinDate}
 						<small class="form-hint form-hint-rule">
 							Date minimale conforme aux règles : {new Date(ruleConformMinDate + 'T00:00:00').toLocaleDateString('fr-FR', { day: 'numeric', month: 'long', year: 'numeric' })}
 						</small>
@@ -1078,6 +1151,16 @@
 	</div>
 {/if}
 
+<!-- Rule Blocked Modal (Hard Mode) -->
+{#if showRuleBlockedModal && ruleBlockedWarnings.length > 0}
+	<RuleBlockedModal
+		warnings={ruleBlockedWarnings}
+		suggestedDates={ruleBlockedSuggestedDates}
+		onSelectDate={handleBlockedSelectDate}
+		onClose={handleBlockedClose}
+	/>
+{/if}
+
 <style>
 	.homework-page {
 		padding: 1.5rem;
@@ -1611,6 +1694,11 @@
 		font-weight: 500;
 	}
 
+	.form-hint-error {
+		color: #dc2626;
+		font-weight: 500;
+	}
+
 	/* Rule override badge */
 	.homework-badges {
 		display: flex;