Combat: GM concentration flag on combatants so the save prompt actually fires

The post-damage concentration save was keyed only off Character.concentration, which is
set just when a seated player casts via their own panel — so a GM running a monster/NPC
(or any unseated PC) never got the reminder, defeating the feature. Combatants now carry
an optional concentrating flag with a one-click toggle in the row; noteConcentrationCheck
keys off it (falling back to the linked Character). Verified in-app: toggle + 10 damage →
'roll a DC 10 Constitution save or lose concentration'.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/features/combat/EncounterTracker.tsx

@@ -2,6 +2,7 @@ import { useEffect, useRef, useState } from 'react';
 import {
   ArrowDown,
   ArrowUp,
+  BrainCircuit,
   ChevronLeft,
   ChevronRight,
   Dices,
@@ -81,11 +82,14 @@ export function EncounterTracker({ encounter, campaign }: { encounter: Encounter
    */
   const noteConcentrationCheck = (combatant: Combatant, damage: number) => {
     if (damage <= 0) return;
+    // The GM-set combatant flag works for any creature (monster/NPC/PC); fall back to
+    // the linked Character's concentration (set when a seated player casts a spell).
     const ch = pcCharacter(combatant);
-    if (!ch?.concentration) return;
+    const spellName = combatant.concentrating || ch?.concentration?.spellName;
+    if (!spellName) return;
     const dc = concentrationDC(damage);
     void encountersRepo.mutate(encounter.id, (e) =>
-      logEvent(e, `${ch.name}: roll a DC ${dc} Constitution save or lose concentration on ${ch.concentration!.spellName}.`));
+      logEvent(e, `${combatant.name}: roll a DC ${dc} Constitution save or lose concentration on ${spellName}.`));
   };
 
   /** Advance the turn; remind (don't roll) when a downed PC's turn begins. */
@@ -481,6 +485,16 @@ function CombatantRow({
               </Badge>
             )}
             <span className="smallcaps" style={{ fontSize: 9 }}>{c.kind}</span>
+            <button
+              type="button"
+              onClick={() => onChange({ concentrating: c.concentrating ? null : 'a spell' })}
+              className={cn('rounded p-0.5', c.concentrating ? 'text-accent' : 'text-faint hover:text-muted')}
+              title={c.concentrating ? `Concentrating on ${c.concentrating} — click to clear` : 'Mark as concentrating (prompts a Con save when damaged)'}
+              aria-label={c.concentrating ? `${c.name} is concentrating; clear` : `Mark ${c.name} as concentrating`}
+              aria-pressed={!!c.concentrating}
+            >
+              <BrainCircuit size={13} aria-hidden />
+            </button>
             {dead && (
               <Badge tone="ember">
                 <Skull size={11} aria-hidden /> DOWN

src/lib/schemas/encounter.ts

@@ -34,6 +34,8 @@ export const combatantSchema = z.object({
   ac: int,
   hp: hpSchema,
   conditions: z.array(conditionSchema).default([]),
+  /** Spell this combatant is concentrating on (GM-set). Drives the post-damage save prompt. */
+  concentrating: z.string().max(120).nullable().optional(),
   notes: z.string().max(2000).default(''),
   /** damage resistances/immunities snapshotted from the bestiary, if known */
   damageDefenses: damageDefensesSchema.optional(),