feat(VocabPracticeDialog, localization): add hard vocabulary marking feature

- Introduced functionality to mark vocabulary items as difficult, allowing users to track and manage challenging words during practice sessions.
- Added buttons for marking and unmarking current vocabulary as hard, enhancing user engagement and learning effectiveness.
- Updated the UI to display the count of hard-marked vocabulary items, providing users with better insights into their learning progress.
- Enhanced localization files for German and English to support new vocabulary marking features, ensuring a consistent user experience across languages.

frontend/src/dialogues/socialnetwork/VocabPracticeDialog.vue

@@ -94,6 +94,12 @@
             <button v-else-if="showSkipButton" @click="skip">
               {{ $t('socialnetwork.vocab.practice.skip') }}
             </button>
+            <button v-if="current && !isCurrentMarkedHard" @click="markCurrentAsHard">
+              {{ $t('socialnetwork.vocab.practice.markHard') }}
+            </button>
+            <button v-else-if="current && isCurrentMarkedHard" @click="unmarkCurrentAsHard">
+              {{ $t('socialnetwork.vocab.practice.unmarkHard') }}
+            </button>
           </div>
 
           <div v-if="lastWrongReview" class="solution-card solution-card--persistent">
@@ -137,6 +143,10 @@
             <span class="k">{{ $t('socialnetwork.vocab.practice.fail') }}</span>
             <span class="v">{{ wrongCount }} ({{ failPercent }}%)</span>
           </div>
+          <div class="statRow">
+            <span class="k">{{ $t('socialnetwork.vocab.practice.hardCount') }}</span>
+            <span class="v">{{ hardCount }}</span>
+          </div>
         </div>
       </div>
     </div>
@@ -188,6 +198,7 @@ export default {
       lastCorrect: false,
       locked: false,
       autoAdvanceTimer: null,
+      hardVocabMap: {}, // { [normalizedPairKey]: { learning, reference, markedAt } }
     };
   },
   computed: {
@@ -277,8 +288,70 @@ export default {
       const total = this.srsTotalDue || 0;
       return Math.max(0, total - this.srsDoneCount);
     },
+    hardCount() {
+      return Object.keys(this.hardVocabMap || {}).length;
+    },
+    isCurrentMarkedHard() {
+      if (!this.current) return false;
+      return Boolean(this.hardVocabMap[this.getHardKey(this.current)]);
+    }
   },
   methods: {
+    hardStorageKey() {
+      const courseId = this.openParams?.courseId;
+      if (!courseId) return null;
+      return `yourpart:vocab:hardList:${courseId}`;
+    },
+    loadHardVocabMap() {
+      const key = this.hardStorageKey();
+      if (!key) {
+        this.hardVocabMap = {};
+        return;
+      }
+      try {
+        const raw = localStorage.getItem(key);
+        const parsed = raw ? JSON.parse(raw) : {};
+        this.hardVocabMap = parsed && typeof parsed === 'object' ? parsed : {};
+      } catch (_) {
+        this.hardVocabMap = {};
+      }
+    },
+    saveHardVocabMap() {
+      const key = this.hardStorageKey();
+      if (!key) return;
+      try {
+        localStorage.setItem(key, JSON.stringify(this.hardVocabMap || {}));
+      } catch (_) {
+        // ignore quota/private-mode issues
+      }
+    },
+    getHardKey(item) {
+      const learning = this.normalize(item?.learning || '');
+      const reference = this.normalize(item?.reference || '');
+      return `${learning}|${reference}`;
+    },
+    markCurrentAsHard() {
+      if (!this.current) return;
+      const key = this.getHardKey(this.current);
+      this.hardVocabMap = {
+        ...this.hardVocabMap,
+        [key]: {
+          learning: this.current.learning,
+          reference: this.current.reference,
+          markedAt: new Date().toISOString()
+        }
+      };
+      this.saveHardVocabMap();
+    },
+    unmarkCurrentAsHard() {
+      if (!this.current) return;
+      const key = this.getHardKey(this.current);
+      if (!this.hardVocabMap[key]) return;
+      const next = { ...this.hardVocabMap };
+      delete next[key];
+      this.hardVocabMap = next;
+      this.saveHardVocabMap();
+    },
     getLocalDateKey() {
       const d = new Date();
       const y = d.getFullYear();
@@ -381,6 +454,7 @@ export default {
       this.lastWrongReview = null;
       this.pendingRetry = null;
       this.pool = [];
+      this.hardVocabMap = {};
       this.locked = false;
       this.resetQuestion();
       this.$refs.dialog.open();
@@ -388,6 +462,7 @@ export default {
         document.addEventListener('keydown', this.handleKeyDown);
       });
       this.reloadPool();
+      this.loadHardVocabMap();
     },
     close() {
       if (this.autoAdvanceTimer) {

frontend/src/i18n/locales/de/socialnetwork.json

@@ -461,6 +461,9 @@
                 "remaining": "Noch offen",
                 "success": "Erfolg",
                 "fail": "Misserfolg",
+                "hardCount": "Schwer markiert",
+                "markHard": "Als schwer markieren",
+                "unmarkHard": "Aus schwer entfernen",
                 "srsRateTitle": "Wie sicher war das?",
                 "srsAgain": "Nochmal",
                 "srsAgainHint": "sehr bald wiederholen",

frontend/src/i18n/locales/en/socialnetwork.json

@@ -461,6 +461,9 @@
                 "remaining": "Remaining",
                 "success": "Success",
                 "fail": "Fail",
+                "hardCount": "Marked difficult",
+                "markHard": "Mark as difficult",
+                "unmarkHard": "Remove difficult mark",
                 "srsRateTitle": "How solid did it feel?",
                 "srsAgain": "Again",
                 "srsAgainHint": "repeat very soon",