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.

pkg/fbs/scrabblefb/StatsView.go

@@ -101,8 +101,28 @@ func (rcv *StatsView) MutateMaxWordPoints(n int32) bool {
 	return rcv._tab.MutateInt32Slot(12, n)
 }
 
+func (rcv *StatsView) BestMoves(obj *BestMoveView, j int) bool {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(14))
+	if o != 0 {
+		x := rcv._tab.Vector(o)
+		x += flatbuffers.UOffsetT(j) * 4
+		x = rcv._tab.Indirect(x)
+		obj.Init(rcv._tab.Bytes, x)
+		return true
+	}
+	return false
+}
+
+func (rcv *StatsView) BestMovesLength() int {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(14))
+	if o != 0 {
+		return rcv._tab.VectorLen(o)
+	}
+	return 0
+}
+
 func StatsViewStart(builder *flatbuffers.Builder) {
-	builder.StartObject(5)
+	builder.StartObject(6)
 }
 func StatsViewAddWins(builder *flatbuffers.Builder, wins int32) {
 	builder.PrependInt32Slot(0, wins, 0)
@@ -119,6 +139,12 @@ func StatsViewAddMaxGamePoints(builder *flatbuffers.Builder, maxGamePoints int32
 func StatsViewAddMaxWordPoints(builder *flatbuffers.Builder, maxWordPoints int32) {
 	builder.PrependInt32Slot(4, maxWordPoints, 0)
 }
+func StatsViewAddBestMoves(builder *flatbuffers.Builder, bestMoves flatbuffers.UOffsetT) {
+	builder.PrependUOffsetTSlot(5, flatbuffers.UOffsetT(bestMoves), 0)
+}
+func StatsViewStartBestMovesVector(builder *flatbuffers.Builder, numElems int) flatbuffers.UOffsetT {
+	return builder.StartVector(4, numElems, 4)
+}
 func StatsViewEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
 	return builder.EndObject()
 }