feat(sign-in): make auth buttons responsive with icon-only mobile affordance

tests/sign-in.test.ts

@@ -0,0 +1,12 @@
+import { describe, expect, test } from "bun:test";
+import { readFileSync } from "node:fs";
+
+describe("sign-in header affordance", () => {
+	test("auth control switches to an icon-only mobile affordance via useIsMobile", () => {
+		const source = readFileSync("src/components/sign-in.tsx", "utf8");
+
+		expect(source).toContain("useIsMobile");
+		expect(source).toContain('size={isMobile ? "icon" : "sm"}');
+		expect(source).toContain('aria-label={session?.user ? "Sign out" : "Sign in"}');
+	});
+});