fix(ux): improve destructive action safety, accents, navigation and feedback

Replace window.confirm() with Modal dialogs for class deletion and grade
reset. Add unsaved-changes guards to AssessmentFormView. Warn before
deleting exercises/elements with existing grades. Surface invalid grades
in a detailed Modal after save. Replace GradingView local toasts with
global notification store. Fix missing French accents across CouncilView,
CouncilStudentDetail, and ConfigView. Make dashboard "À corriger" card
and student list rows clickable. Add visual hierarchy to assessment
detail actions. Add loading overlay to trimester switch. Simplify email
sending workflow by removing mode-switch pattern.

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

frontend/src/views/CouncilView.vue

@@ -66,14 +66,14 @@
             @next="navigateStudent(1)"
           />
           <div v-else class="bg-white rounded-xl shadow-md p-12 text-center">
-            <p class="text-gray-400">Selectionnez un eleve dans la liste</p>
+            <p class="text-gray-400">Sélectionnez un élève dans la liste</p>
           </div>
         </div>
       </div>
 
       <!-- Empty state -->
       <div v-else-if="currentStats" class="flex-1 flex items-center justify-center">
-        <p class="text-gray-500">Aucune donnee disponible pour le trimestre {{ trimester }}</p>
+        <p class="text-gray-500">Aucune donnée disponible pour le trimestre {{ trimester }}</p>
       </div>
     </template>
   </div>