feat(offline): hotseat creation roster + host-participate flow

NewGame offline 'with friends' now builds a local pass-and-play game:
- lib/roster.ts: keep-last-valid name + roster->seats (unit-tested).
- NewGame.svelte: master host-PIN gate (rows disabled until set),
  'are you playing too?' prompt seating the host at row 0, 2-4 player
  rows with inline name validation + optional per-seat PIN, add/remove
  (remove behind the master PIN), variant + multiple-words, Start.
- PinPad: verification via a verify(pin) callback (UI-held lock at
  creation, source-held in-game) instead of a lock prop.
- i18n: hotseat.* (en + ru); the offline mode selector is now shown offline.

ui/src/lib/roster.ts

@@ -0,0 +1,32 @@
+// Pure helpers for the offline hotseat (pass-and-play) creation roster: keeping a last-valid name as
+// a row is edited, and turning the finished roster into engine seats. Kept out of the component so it
+// is unit-testable; the form itself is exercised by the e2e layer.
+
+import { validDisplayName } from './profileValidation';
+import type { PinLock } from './pin';
+import type { Seat } from './localgame/serialize';
+
+/** RosterRow is one editable player row: the live input, the last valid name kept when the input
+ *  goes invalid, and the optional per-seat PIN lock. */
+export interface RosterRow {
+  name: string;
+  committed: string;
+  pin?: PinLock;
+}
+
+/** commitName returns the new last-valid name: the trimmed input when it is a valid display name,
+ *  otherwise prevCommitted unchanged — so editing a set name into an invalid one keeps the old one. */
+export function commitName(input: string, prevCommitted: string): string {
+  return validDisplayName(input) ? input.trim() : prevCommitted;
+}
+
+/** rosterReady reports whether the roster can start a game: 2-4 rows, each with a committed name. */
+export function rosterReady(rows: RosterRow[]): boolean {
+  return rows.length >= 2 && rows.length <= 4 && rows.every((r) => r.committed !== '');
+}
+
+/** buildSeats turns committed roster rows into engine seats — all human, account-less (local
+ *  players sharing a device), carrying each row's optional PIN. */
+export function buildSeats(rows: RosterRow[]): Seat[] {
+  return rows.map((r): Seat => ({ kind: 'human', name: r.committed, pin: r.pin }));
+}