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/mock/data.ts

@@ -113,7 +113,7 @@ export function mockInvitations(): Invitation[] {
       hintsAllowed: true,
       hintsPerPlayer: 1,
       multipleWordsPerTurn: false,
-      dropoutTiles: 'remove',
+            dropoutTiles: 'remove',
       status: 'pending',
       gameId: '',
       expiresAtUnix: Math.floor(Date.now() / 1000) + 7 * 86400,
@@ -198,6 +198,7 @@ function activeGame(): MockGame {
       toMove: 0,
       turnTimeoutSecs: 86400,
       multipleWordsPerTurn: true,
+      noRepeatWords: false,
       moveCount: G1_MOVES.length,
       endReason: '',
       lastActivityUnix: Math.floor(Date.now() / 1000) - 7200,
@@ -237,6 +238,7 @@ function finishedG2(): MockGame {
       toMove: 0,
       turnTimeoutSecs: 86400,
       multipleWordsPerTurn: true,
+      noRepeatWords: false,
       moveCount: 2,
       endReason: 'normal',
       lastActivityUnix: Math.floor(Date.now() / 1000) - 86400,
@@ -277,6 +279,7 @@ function finishedG3(): MockGame {
       toMove: 0,
       turnTimeoutSecs: 86400,
       multipleWordsPerTurn: false,
+      noRepeatWords: false,
       moveCount: 1,
       endReason: 'resignation',
       lastActivityUnix: Math.floor(Date.now() / 1000) - 172800,