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/i18n/en.ts

@@ -398,6 +398,16 @@ export const en = {
   'pin.wrong': 'Wrong PIN',
   'pin.setNew': 'Set a new PIN',
   'pin.remove': 'Remove PIN',
+
+  'hotseat.hostPin': 'Host password',
+  'hotseat.setPin': 'Password',
+  'hotseat.changePin': 'Change',
+  'hotseat.playerName': 'Player name',
+  'hotseat.addPlayer': 'Add player',
+  'hotseat.removePlayer': 'Remove player',
+  'hotseat.hostPlaysTitle': 'Are you playing too?',
+  'hotseat.hostPlaysYes': 'Yes, I play',
+  'hotseat.hostPlaysNo': 'No',
 } as const;
 
 export type MessageKey = keyof typeof en;