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/__tests__/roll-initiative.test.ts

@@ -1,5 +1,5 @@
 import { describe, expect, it } from "vitest";
-import { rollInitiative } from "../roll-initiative.js";
+import { rollInitiative, selectRoll } from "../roll-initiative.js";
 import { isDomainError } from "../types.js";
 import { expectDomainError } from "./test-helpers.js";
 
@@ -63,3 +63,31 @@ describe("rollInitiative", () => {
 		});
 	});
 });
+
+describe("selectRoll", () => {
+	it("normal mode returns the first roll", () => {
+		expect(selectRoll(8, 15, "normal")).toBe(8);
+	});
+
+	it("advantage returns the higher roll", () => {
+		expect(selectRoll(8, 15, "advantage")).toBe(15);
+	});
+
+	it("advantage returns the higher roll (reversed)", () => {
+		expect(selectRoll(15, 8, "advantage")).toBe(15);
+	});
+
+	it("disadvantage returns the lower roll", () => {
+		expect(selectRoll(8, 15, "disadvantage")).toBe(8);
+	});
+
+	it("disadvantage returns the lower roll (reversed)", () => {
+		expect(selectRoll(15, 8, "disadvantage")).toBe(8);
+	});
+
+	it("equal rolls return the same value for all modes", () => {
+		expect(selectRoll(12, 12, "normal")).toBe(12);
+		expect(selectRoll(12, 12, "advantage")).toBe(12);
+		expect(selectRoll(12, 12, "disadvantage")).toBe(12);
+	});
+});