Add RFC 6238 one-time codes

WebUntis authenticates the mobile app not with a password but with a static
base32 key, from which every request derives a fresh code. That is the
credential an always-on server wants: it works under the school's SSO, needs
no session, and expires only when a new key is generated.

Implemented rather than pulled in. It is HMAC-SHA1 plus a truncation,
node:crypto has the hard part, and a dependency that handles a credential is
one worth not having. The tests are the RFC's own vectors, which validate the
base32 table as much as the arithmetic.

The code comes back zero-padded, as a string. One in ten begins with a zero
and a JSON number would drop it, which is a login that fails a tenth of the
time and looks like a server fault.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

test/totp.test.ts

@@ -0,0 +1,59 @@
+import assert from 'node:assert/strict';
+import { describe, it } from 'node:test';
+import { base32Decode, totp } from '../src/core/totp.ts';
+
+/**
+ * RFC 6238's own SHA-1 vectors, whose key is the ASCII string
+ * "12345678901234567890" — base32 below. They validate the decode as much as
+ * the code: a wrong base32 table would not reproduce a single one of them.
+ */
+const RFC_KEY = 'GEZDGNBVGY3TQOJQGEZDGNBVGY3TQOJQ';
+
+describe('totp', () => {
+	it('reproduces the RFC 6238 vectors', () => {
+		// Each vector's 8-digit code, truncated to the 6 digits WebUntis wants.
+		const vectors: [seconds: number, code: string][] = [
+			[59, '287082'],
+			[1_111_111_109, '081804'],
+			[1_111_111_111, '050471'],
+			[1_234_567_890, '005924'],
+			[2_000_000_000, '279037'],
+			[20_000_000_000, '353130'],
+		];
+		for (const [seconds, code] of vectors) {
+			assert.equal(totp(RFC_KEY, seconds * 1000), code, `at ${seconds}s`);
+		}
+	});
+
+	it('keeps a leading zero, which is why the code travels as a string', () => {
+		const code = totp(RFC_KEY, 1_234_567_890_000);
+		assert.equal(code, '005924');
+		assert.equal(code.length, 6);
+		// The bug this guards: sending the code as a JSON number would make it 5924.
+		assert.notEqual(String(Number(code)), code);
+	});
+
+	it('changes with the 30-second step and not within it', () => {
+		const base = 1_700_000_000_000;
+		assert.equal(totp(RFC_KEY, base), totp(RFC_KEY, base + 29_000 - (base % 30_000)));
+		assert.notEqual(totp(RFC_KEY, base), totp(RFC_KEY, base + 30_000));
+	});
+});
+
+describe('base32Decode', () => {
+	it('decodes to the bytes behind the RFC key', () => {
+		assert.equal(base32Decode(RFC_KEY).toString('utf8'), '12345678901234567890');
+	});
+
+	it('accepts lowercase, padding and spaces, as a copied key arrives', () => {
+		const expected = base32Decode('JBSWY3DP');
+		assert.deepEqual(base32Decode('jbswy3dp'), expected);
+		assert.deepEqual(base32Decode('JBSW Y3DP'), expected);
+		assert.deepEqual(base32Decode('JBSWY3DP===='), expected);
+	});
+
+	it('refuses a value that is not base32 rather than deriving a wrong code', () => {
+		assert.throws(() => base32Decode('nope!'), /outside A-Z and 2-7/);
+		assert.throws(() => base32Decode('   '), /empty/);
+	});
+});