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/BestMoveView.go

@@ -0,0 +1,101 @@
+// Code generated by the FlatBuffers compiler. DO NOT EDIT.
+
+package scrabblefb
+
+import (
+	flatbuffers "github.com/google/flatbuffers/go"
+)
+
+type BestMoveView struct {
+	_tab flatbuffers.Table
+}
+
+func GetRootAsBestMoveView(buf []byte, offset flatbuffers.UOffsetT) *BestMoveView {
+	n := flatbuffers.GetUOffsetT(buf[offset:])
+	x := &BestMoveView{}
+	x.Init(buf, n+offset)
+	return x
+}
+
+func FinishBestMoveViewBuffer(builder *flatbuffers.Builder, offset flatbuffers.UOffsetT) {
+	builder.Finish(offset)
+}
+
+func GetSizePrefixedRootAsBestMoveView(buf []byte, offset flatbuffers.UOffsetT) *BestMoveView {
+	n := flatbuffers.GetUOffsetT(buf[offset+flatbuffers.SizeUint32:])
+	x := &BestMoveView{}
+	x.Init(buf, n+offset+flatbuffers.SizeUint32)
+	return x
+}
+
+func FinishSizePrefixedBestMoveViewBuffer(builder *flatbuffers.Builder, offset flatbuffers.UOffsetT) {
+	builder.FinishSizePrefixed(offset)
+}
+
+func (rcv *BestMoveView) Init(buf []byte, i flatbuffers.UOffsetT) {
+	rcv._tab.Bytes = buf
+	rcv._tab.Pos = i
+}
+
+func (rcv *BestMoveView) Table() flatbuffers.Table {
+	return rcv._tab
+}
+
+func (rcv *BestMoveView) Variant() []byte {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(4))
+	if o != 0 {
+		return rcv._tab.ByteVector(o + rcv._tab.Pos)
+	}
+	return nil
+}
+
+func (rcv *BestMoveView) Score() int32 {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(6))
+	if o != 0 {
+		return rcv._tab.GetInt32(o + rcv._tab.Pos)
+	}
+	return 0
+}
+
+func (rcv *BestMoveView) MutateScore(n int32) bool {
+	return rcv._tab.MutateInt32Slot(6, n)
+}
+
+func (rcv *BestMoveView) Word(obj *BestMoveTile, j int) bool {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(8))
+	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 *BestMoveView) WordLength() int {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(8))
+	if o != 0 {
+		return rcv._tab.VectorLen(o)
+	}
+	return 0
+}
+
+func BestMoveViewStart(builder *flatbuffers.Builder) {
+	builder.StartObject(3)
+}
+func BestMoveViewAddVariant(builder *flatbuffers.Builder, variant flatbuffers.UOffsetT) {
+	builder.PrependUOffsetTSlot(0, flatbuffers.UOffsetT(variant), 0)
+}
+func BestMoveViewAddScore(builder *flatbuffers.Builder, score int32) {
+	builder.PrependInt32Slot(1, score, 0)
+}
+func BestMoveViewAddWord(builder *flatbuffers.Builder, word flatbuffers.UOffsetT) {
+	builder.PrependUOffsetTSlot(2, flatbuffers.UOffsetT(word), 0)
+}
+func BestMoveViewStartWordVector(builder *flatbuffers.Builder, numElems int) flatbuffers.UOffsetT {
+	return builder.StartVector(4, numElems, 4)
+}
+func BestMoveViewEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
+	return builder.EndObject()
+}