fix: level-up formula uses current level, add xpToNextLevel to API

XP threshold was computed on level+1 (target), making early levels too
steep (283 XP for level 2 instead of 100). Now uses current level:
level 1→2 = 100 XP, level 2→3 = 283 XP, level 10→11 = 3162 XP.

Added xpToNextLevel field to character and combat responses so the
frontend can display accurate progress bars.

src/character/character.service.ts

@@ -11,6 +11,7 @@ import { LevelThreshold } from './entities/level-threshold.entity';
 import { CreateCharacterDto } from './dto/create-character.dto';
 import { DistributeStatsDto } from './dto/distribute-stats.dto';
 import { User } from '../user/user.entity';
+import { xpRequiredForLevel } from '../combat/combat.engine';
 
 const STAT_POOL = 10; // 5 stats × 1 base + 5 points à distribuer
 const ENDURANCE_REGEN_MINUTES = 6; // 1 pt d'endurance toutes les 6 min = 10 pts/heure
@@ -35,7 +36,7 @@ export class CharacterService {
     return Math.min(character.enduranceSaved + recharge, character.enduranceMax);
   }
 
-  async create(dto: CreateCharacterDto, user: User): Promise<Character & { enduranceCurrent: number }> {
+  async create(dto: CreateCharacterDto, user: User) {
     const totalStats =
       dto.force + dto.agilite + dto.intelligence + dto.chance + dto.vitalite;
 
@@ -66,10 +67,14 @@ export class CharacterService {
     });
 
     const saved = await this.characterRepository.save(character);
-    return { ...saved, enduranceCurrent: this.calculateEndurance(saved) };
+    return {
+      ...saved,
+      enduranceCurrent: this.calculateEndurance(saved),
+      xpToNextLevel: xpRequiredForLevel(saved.level),
+    };
   }
 
-  async findByUser(user: User): Promise<Character & { enduranceCurrent: number }> {
+  async findByUser(user: User) {
     const character = await this.characterRepository.findOne({
       where: { userId: user.id },
     });
@@ -78,7 +83,11 @@ export class CharacterService {
       throw new NotFoundException('Aucun personnage trouvé pour ce joueur');
     }
 
-    return { ...character, enduranceCurrent: this.calculateEndurance(character) };
+    return {
+      ...character,
+      enduranceCurrent: this.calculateEndurance(character),
+      xpToNextLevel: xpRequiredForLevel(character.level),
+    };
   }
 
   async getEndurance(