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/combat/combat.service.ts

@@ -14,6 +14,7 @@ import { CommunityService } from '../community/community.service';
 import {
   resolveCombat,
   applyXpGain,
+  xpRequiredForLevel,
   CombatantStats,
 } from './combat.engine';
 
@@ -237,6 +238,7 @@ export class CombatService {
         character: {
           level: character.level,
           xp: character.xp,
+          xpToNextLevel: xpRequiredForLevel(character.level),
           gold: character.gold,
           hpCurrent: character.hpCurrent,
           hpMax: character.hpMax,