feat(stats): show the best move word per game variant

Replace the single "best move" number on the statistics screen with a
full-width per-variant breakdown: the highest-scoring play in each variant
the player has played, drawn as game tiles (a wildcard shows its letter but
no value), with the words and scores right-aligned to shared edges.

- backend: new account_best_move table (PK account_id+variant) keeping the
  main word as JSON tiles {letter,value,blank}; captured at game finish in
  buildStats (blank flags taken from every placed blank — equivalent to the
  final board), upserted in the finish transaction and replaced only by a
  strictly higher-scoring play. Guest/honest-AI games still record nothing.
  GetStats + statsDTO expose best_moves.
- wire: StatsView gains best_moves:[BestMoveView{variant,score,word:[BestMoveTile]}]
  (trailing, backward-compatible); gateway encodeStats + UI codec updated.
- ui: new WordTiles component (board's tile look, fixed px size); Stats.svelte
  drops the maxWord card and adds the full-width best-move card (catalogue
  order, empty variants omitted).
- docs: ARCHITECTURE §9 + schema, FUNCTIONAL (+ru), UI_DESIGN.

Tests: mainWordTiles unit + buildStats end-to-end (inttest) + gateway and UI
codec round-trips (incl. a blank tile) + e2e.

gateway/internal/transcode/encode_social.go

@@ -91,15 +91,61 @@ func encodeRedeemResult(r backendclient.RedeemResultResp) []byte {
 	return b.FinishedBytes()
 }
 
-// encodeStats builds a StatsView payload.
+// buildBestMoveTile builds a BestMoveTile table and returns its offset.
+func buildBestMoveTile(b *flatbuffers.Builder, t backendclient.BestMoveTileResp) flatbuffers.UOffsetT {
+	letter := b.CreateString(t.Letter)
+	fb.BestMoveTileStart(b)
+	fb.BestMoveTileAddLetter(b, letter)
+	fb.BestMoveTileAddValue(b, int32(t.Value))
+	fb.BestMoveTileAddBlank(b, t.Blank)
+	return fb.BestMoveTileEnd(b)
+}
+
+// buildBestMove builds a BestMoveView table — its word tile vector first, per the
+// bottom-up rule — and returns its offset.
+func buildBestMove(b *flatbuffers.Builder, m backendclient.BestMoveResp) flatbuffers.UOffsetT {
+	tiles := make([]flatbuffers.UOffsetT, len(m.Word))
+	for i, t := range m.Word {
+		tiles[i] = buildBestMoveTile(b, t)
+	}
+	fb.BestMoveViewStartWordVector(b, len(tiles))
+	for i := len(tiles) - 1; i >= 0; i-- {
+		b.PrependUOffsetT(tiles[i])
+	}
+	word := b.EndVector(len(tiles))
+	variant := b.CreateString(m.Variant)
+	fb.BestMoveViewStart(b)
+	fb.BestMoveViewAddVariant(b, variant)
+	fb.BestMoveViewAddScore(b, int32(m.Score))
+	fb.BestMoveViewAddWord(b, word)
+	return fb.BestMoveViewEnd(b)
+}
+
+// encodeStats builds a StatsView payload, embedding the per-variant best moves (each a
+// BestMoveView with its word tiles) when present.
 func encodeStats(r backendclient.StatsResp) []byte {
-	b := flatbuffers.NewBuilder(64)
+	b := flatbuffers.NewBuilder(256)
+	var bestMoves flatbuffers.UOffsetT
+	if len(r.BestMoves) > 0 {
+		offs := make([]flatbuffers.UOffsetT, len(r.BestMoves))
+		for i, m := range r.BestMoves {
+			offs[i] = buildBestMove(b, m)
+		}
+		fb.StatsViewStartBestMovesVector(b, len(offs))
+		for i := len(offs) - 1; i >= 0; i-- {
+			b.PrependUOffsetT(offs[i])
+		}
+		bestMoves = b.EndVector(len(offs))
+	}
 	fb.StatsViewStart(b)
 	fb.StatsViewAddWins(b, int32(r.Wins))
 	fb.StatsViewAddLosses(b, int32(r.Losses))
 	fb.StatsViewAddDraws(b, int32(r.Draws))
 	fb.StatsViewAddMaxGamePoints(b, int32(r.MaxGamePoints))
 	fb.StatsViewAddMaxWordPoints(b, int32(r.MaxWordPoints))
+	if len(r.BestMoves) > 0 {
+		fb.StatsViewAddBestMoves(b, bestMoves)
+	}
 	b.Finish(fb.StatsViewEnd(b))
 	return b.FinishedBytes()
 }