feat(rules): forbid repeating a word already on the board in Erudit

Russian "Эрудит" treats a word laid on the board as belonging to the
game: it cannot be laid again. Neither the solver, the backend nor the
offline JS port knew the rule, so a player (and the robot) could replay a
word freely. Official Scrabble places no such restriction, so both
Scrabble variants keep playing unrestricted.

The rule applies in two ways. A play whose main word is already on the
board is illegal, and is neither accepted nor generated. A play whose
perpendicular cross-word is already there stands — that word is
incidental to laying the main word — but scores nothing. The set of
played words is the game's own move journal, main words and cross-words
alike, compared decoded, so a word spelled with a blank is the same word.

It lives in the game layer, not the solver: only a game knows its
history, and the solver stays stateless and standard-rules. The backend
applies it at submit, at the move preview and over generated moves
(filtering and re-ranking them, so neither the robot nor the hint can
offer a play the engine would then refuse); the client port does the same
for the offline engine and for the on-device preview of an online game.

The rule is pinned per game (games.no_repeat_words, set from the variant
at creation) rather than keyed on the variant, because a game is replayed
from its journal on every open. Applied retroactively it would make an
already-played repeat illegal — closing that game as a draw — and would
rescore a play whose cross-word repeats an earlier word, shifting a live
game's totals. Games created before the rule keep playing without it. The
flag rides the wire as a trailing field because the client's preview must
score the way the server does, and offline games pin the same answer in
their own record.

ui/src/lib/localgame/source.ts

@@ -18,6 +18,7 @@ import { HINT_GATE_MS } from '../hints';
 import { LOCAL_ID_PREFIX, isLocalGameId } from './id';
 import { decide } from '../robot/strategy';
 import { verifyPin, type PinLock } from '../pin';
+import { noRepeatWordsFor } from '../variants';
 import { GatewayError, type PlacedTile } from '../client';
 import type {
   EvalResult,
@@ -122,6 +123,7 @@ export class LocalSource implements GameLoopSource {
       players: opts.seats.length,
       dawg,
       multipleWords: opts.multipleWords,
+      noRepeatWords: noRepeatWordsFor(opts.variant),
     });
     const now = nowUnix();
     const record = serializeGame(game, {
@@ -483,6 +485,7 @@ export class LocalSource implements GameLoopSource {
       toMove: game.currentPlayer,
       turnTimeoutSecs: 0,
       multipleWordsPerTurn: record.multipleWords,
+      noRepeatWords: !!record.noRepeatWords,
       moveCount: game.moveCount,
       endReason: game.isOver ? game.endReason : '',
       lastActivityUnix: record.updatedAtUnix,