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/codec.test.ts

@@ -423,6 +423,7 @@ describe('codec', () => {
     fb.GameView.addVsAi(b, true);
     fb.GameView.addUnreadChat(b, true);
     fb.GameView.addKind(b, 2);
+    fb.GameView.addNoRepeatWords(b, true);
     const game = fb.GameView.endGameView(b);
     const games = fb.GameList.createGamesVector(b, [game]);
     fb.GameList.startGameList(b);
@@ -441,6 +442,9 @@ describe('codec', () => {
     expect(gl.games[0].vsAi).toBe(true);
     expect(gl.games[0].unreadChat).toBe(true);
     expect(gl.games[0].kind).toBe(2);
+    // The Erudit no-repeat rule is pinned per game and rides the view: the on-device move
+    // preview needs it to score the way the server does.
+    expect(gl.games[0].noRepeatWords).toBe(true);
     expect(gl.atGameLimit).toBe(true);
   });