Add advantage/disadvantage rolling for initiative

Right-click or long-press the d20 button (per-combatant or Roll All)
to open a context menu with Advantage and Disadvantage options.
Normal left-click behavior is unchanged.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/domain/src/roll-initiative.ts

@@ -1,5 +1,21 @@
 import type { DomainError } from "./types.js";
 
+export type RollMode = "normal" | "advantage" | "disadvantage";
+
+/**
+ * Selects the effective roll from two dice values based on the roll mode.
+ * Advantage takes the higher, disadvantage takes the lower.
+ */
+export function selectRoll(
+	roll1: number,
+	roll2: number,
+	mode: RollMode,
+): number {
+	if (mode === "advantage") return Math.max(roll1, roll2);
+	if (mode === "disadvantage") return Math.min(roll1, roll2);
+	return roll1;
+}
+
 /**
  * Pure function that computes initiative from a resolved dice roll and modifier.
  * The dice roll must be an integer in [1, 20].