Fixes: adv/dis as a global toggle, auto-number duplicate combatants

- Advantage/Disadvantage are now toggle modes (rollStore.mode) applied to d20
  rolls everywhere (dice page, sheet skills/saves/attacks), preserving modifiers;
  applyRollMode() rewrites the first d20 -> 2d20kh1/kl1
- addCombatant auto-numbers duplicate names (Goblin 1, Goblin 2, …); the first
  duplicate also renames the original. Quantity add relies on this.
- Tests for applyRollMode + duplicate numbering

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/stores/rollStore.ts

@@ -1,5 +1,5 @@
 import { create } from 'zustand';
-import type { RollResult } from '@/lib/dice/notation';
+import type { RollResult, RollMode } from '@/lib/dice/notation';
 import type { Degree } from '@/lib/dice/check';
 
 export interface TrayRoll {
@@ -13,13 +13,21 @@ export interface TrayRoll {
 
 interface RollState {
   last: TrayRoll | null;
+  /** advantage/disadvantage toggle applied to d20 rolls everywhere */
+  mode: RollMode;
   push: (roll: Omit<TrayRoll, 'seq'>) => void;
   dismiss: () => void;
+  setMode: (mode: RollMode) => void;
+  /** click the same mode again to turn it off */
+  toggleMode: (mode: Exclude<RollMode, 'normal'>) => void;
 }
 
 let seq = 0;
-export const useRollStore = create<RollState>((set) => ({
+export const useRollStore = create<RollState>((set, get) => ({
   last: null,
+  mode: 'normal',
   push: (roll) => set({ last: { ...roll, seq: ++seq } }),
   dismiss: () => set({ last: null }),
+  setMode: (mode) => set({ mode }),
+  toggleMode: (mode) => set({ mode: get().mode === mode ? 'normal' : mode }),
 }));