feat(offline): PIN lock primitives + Apple-style keypad

Groundwork for offline pass-and-play (hotseat) seat/host PIN locks:
- lib/pin.ts: salted SHA-256 PinLock (newSalt/hashPin/newLock/verifyPin);
  a social lock for a shared device (documented, not cryptography).
- components/PinPad.svelte: 4-digit keypad (set/verify/change), auto-submit
  on the 4th digit, shake on mismatch, hardware-keyboard support.
- i18n: pin.* keys (en + ru).

Engine/source/UI wiring follows.

ui/src/lib/i18n/en.ts

@@ -389,6 +389,15 @@ export const en = {
   'error.game_active': 'Available only after the game is finished.',
   'error.invalid_profile': 'Some profile fields are invalid.',
   'error.already_confirmed': 'This email is already confirmed.',
+
+  'pin.enter': 'Enter PIN',
+  'pin.create': 'Create a PIN',
+  'pin.repeat': 'Repeat the PIN',
+  'pin.enterCurrent': 'Enter current PIN',
+  'pin.mismatch': "PINs don't match",
+  'pin.wrong': 'Wrong PIN',
+  'pin.setNew': 'Set a new PIN',
+  'pin.remove': 'Remove PIN',
 } as const;
 
 export type MessageKey = keyof typeof en;

ui/src/lib/i18n/ru.ts

@@ -389,4 +389,13 @@ export const ru: Record<MessageKey, string> = {
   'error.game_active': 'Доступно только после завершения игры.',
   'error.invalid_profile': 'Некоторые поля профиля некорректны.',
   'error.already_confirmed': 'Эта почта уже подтверждена.',
+
+  'pin.enter': 'Введите PIN',
+  'pin.create': 'Придумайте PIN',
+  'pin.repeat': 'Повторите PIN',
+  'pin.enterCurrent': 'Введите текущий PIN',
+  'pin.mismatch': 'PIN не совпадает',
+  'pin.wrong': 'Неверный PIN',
+  'pin.setNew': 'Задать новый PIN',
+  'pin.remove': 'Удалить пароль',
 };

ui/src/lib/pin.test.ts

@@ -0,0 +1,49 @@
+import { describe, expect, it } from 'vitest';
+import { hashPin, newLock, newSalt, verifyPin } from './pin';
+
+describe('hashPin', () => {
+  it('is deterministic for the same pin and salt', async () => {
+    const salt = 'fixedsalt';
+    expect(await hashPin('1234', salt)).toBe(await hashPin('1234', salt));
+  });
+
+  it('differs when the salt differs', async () => {
+    expect(await hashPin('1234', 'saltA')).not.toBe(await hashPin('1234', 'saltB'));
+  });
+
+  it('differs when the pin differs', async () => {
+    const salt = 'fixedsalt';
+    expect(await hashPin('1234', salt)).not.toBe(await hashPin('4321', salt));
+  });
+});
+
+describe('newSalt', () => {
+  it('returns a non-empty value', () => {
+    expect(newSalt().length).toBeGreaterThan(0);
+  });
+
+  it('returns a fresh value each call', () => {
+    expect(newSalt()).not.toBe(newSalt());
+  });
+});
+
+describe('newLock / verifyPin', () => {
+  it('produces a lock that verifies its own pin', async () => {
+    const lock = await newLock('1234');
+    expect(lock.hash.length).toBeGreaterThan(0);
+    expect(lock.salt.length).toBeGreaterThan(0);
+    expect(await verifyPin('1234', lock)).toBe(true);
+  });
+
+  it('rejects a wrong pin', async () => {
+    const lock = await newLock('1234');
+    expect(await verifyPin('0000', lock)).toBe(false);
+  });
+
+  it('salts each lock independently (same pin, different hashes)', async () => {
+    const a = await newLock('1234');
+    const b = await newLock('1234');
+    expect(a.salt).not.toBe(b.salt);
+    expect(a.hash).not.toBe(b.hash);
+  });
+});

ui/src/lib/pin.ts

@@ -0,0 +1,45 @@
+// Seat / host PIN locks for offline pass-and-play (hotseat) games.
+//
+// A PinLock is a salted SHA-256 digest of a 4-digit PIN. This is a SOCIAL lock for a
+// shared device — not cryptographic protection. A 4-digit PIN has only 10^4
+// combinations and the whole game runs on the client, so a determined peek reads the
+// racks straight from memory regardless. The salt only keeps the PIN itself out of a
+// casual devtools glance and defeats precomputed tables; it does not make the PIN
+// brute-force resistant.
+
+/** PinLock is a stored, salted SHA-256 digest of a PIN (see the file header caveat). */
+export interface PinLock {
+  hash: string;
+  salt: string;
+}
+
+// b64url encodes bytes as base64url without padding (matches the idiom in vkid.ts).
+function b64url(bytes: Uint8Array): string {
+  let s = '';
+  for (const b of bytes) s += String.fromCharCode(b);
+  return btoa(s).replace(/\+/g, '-').replace(/\//g, '_').replace(/=+$/, '');
+}
+
+/** newSalt returns a fresh random base64url salt for a PinLock. */
+export function newSalt(): string {
+  const a = new Uint8Array(16);
+  crypto.getRandomValues(a);
+  return b64url(a);
+}
+
+/** hashPin returns the base64url SHA-256 digest binding salt to pin. */
+export async function hashPin(pin: string, salt: string): Promise<string> {
+  const digest = await crypto.subtle.digest('SHA-256', new TextEncoder().encode(`${salt}:${pin}`));
+  return b64url(new Uint8Array(digest));
+}
+
+/** newLock builds a PinLock for pin with a fresh salt. */
+export async function newLock(pin: string): Promise<PinLock> {
+  const salt = newSalt();
+  return { salt, hash: await hashPin(pin, salt) };
+}
+
+/** verifyPin reports whether pin matches the digest stored in lock. */
+export async function verifyPin(pin: string, lock: PinLock): Promise<boolean> {
+  return (await hashPin(pin, lock.salt)) === lock.hash;
+}