feat(combat): rejoin the running combat instead of dead-ending

Attacking while a combat is already active returned COMBAT_ALREADY_ACTIVE
and left the hunt page showing an error the player could not act on, with
no way back into the fight they were already in.

Add GET /api/combats/active so the client can resolve that combat, and
have the hunt page navigate into it when an attack is rejected for this
reason. CombatStore now also exposes the error code so callers can tell
this case apart from a genuinely failed attack.

apps/api/src/combat/combat.controller.ts

@@ -7,6 +7,12 @@ import { CombatService } from './combat.service';
 export class CombatController {
   constructor(private readonly combatService: CombatService) {}
 
+  // Declared before ':combatId' so the literal segment wins the route match.
+  @Get('active')
+  getActiveCombat() {
+    return this.combatService.getActiveCombat(DEMO_CHARACTER_ID);
+  }
+
   @Get(':combatId')
   getCombat(@Param('combatId') combatId: string) {
     return this.combatService.getCombat(DEMO_CHARACTER_ID, combatId);