feat(ui): single-word rule indicators + auto-match select redesign

Surface the per-game "single word" rule to the client and refine the
random-opponent New Game screen.

- Wire: thread multiple_words_per_turn into the GameView and Invitation
  FlatBuffers tables (Go + TS regenerated), through pkg/wire builders and both
  the backend push-event and gateway REST paths.
- In-game indicators (single-word games only): a small 1 in the status bar's
  score-preview slot (yields to the live preview) and a centred "One word per
  turn" label in the history-drawer header. Standard games show neither.
- Invitation card gains a "One word per turn" line for single-word invitations.
- Auto-match redesign: variant plaques are mutually-exclusive selects (highlight
  on tap, no longer enqueue); a lone offered variant is pre-selected; a bottom
  "Start game" button (disabled until a variant is chosen) confirms. The rule
  toggle appears once a Russian variant is selected.
- Tests: e2e for the new auto flow and the in-game indicator (mock g3 is a
  single-word game); mock/data + fixtures carry the new field. Docs: UI_DESIGN.

pkg/fbs/scrabblefb/Invitation.go

@@ -126,15 +126,19 @@ func (rcv *Invitation) MutateHintsPerPlayer(n int32) bool {
 	return rcv._tab.MutateInt32Slot(16, n)
 }
 
-func (rcv *Invitation) DropoutTiles() []byte {
+func (rcv *Invitation) MultipleWordsPerTurn() bool {
 	o := flatbuffers.UOffsetT(rcv._tab.Offset(18))
 	if o != 0 {
-		return rcv._tab.ByteVector(o + rcv._tab.Pos)
+		return rcv._tab.GetBool(o + rcv._tab.Pos)
 	}
-	return nil
+	return false
 }
 
-func (rcv *Invitation) Status() []byte {
+func (rcv *Invitation) MutateMultipleWordsPerTurn(n bool) bool {
+	return rcv._tab.MutateBoolSlot(18, n)
+}
+
+func (rcv *Invitation) DropoutTiles() []byte {
 	o := flatbuffers.UOffsetT(rcv._tab.Offset(20))
 	if o != 0 {
 		return rcv._tab.ByteVector(o + rcv._tab.Pos)
@@ -142,7 +146,7 @@ func (rcv *Invitation) Status() []byte {
 	return nil
 }
 
-func (rcv *Invitation) GameId() []byte {
+func (rcv *Invitation) Status() []byte {
 	o := flatbuffers.UOffsetT(rcv._tab.Offset(22))
 	if o != 0 {
 		return rcv._tab.ByteVector(o + rcv._tab.Pos)
@@ -150,8 +154,16 @@ func (rcv *Invitation) GameId() []byte {
 	return nil
 }
 
-func (rcv *Invitation) ExpiresAtUnix() int64 {
+func (rcv *Invitation) GameId() []byte {
 	o := flatbuffers.UOffsetT(rcv._tab.Offset(24))
+	if o != 0 {
+		return rcv._tab.ByteVector(o + rcv._tab.Pos)
+	}
+	return nil
+}
+
+func (rcv *Invitation) ExpiresAtUnix() int64 {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(26))
 	if o != 0 {
 		return rcv._tab.GetInt64(o + rcv._tab.Pos)
 	}
@@ -159,11 +171,11 @@ func (rcv *Invitation) ExpiresAtUnix() int64 {
 }
 
 func (rcv *Invitation) MutateExpiresAtUnix(n int64) bool {
-	return rcv._tab.MutateInt64Slot(24, n)
+	return rcv._tab.MutateInt64Slot(26, n)
 }
 
 func InvitationStart(builder *flatbuffers.Builder) {
-	builder.StartObject(11)
+	builder.StartObject(12)
 }
 func InvitationAddId(builder *flatbuffers.Builder, id flatbuffers.UOffsetT) {
 	builder.PrependUOffsetTSlot(0, flatbuffers.UOffsetT(id), 0)
@@ -189,17 +201,20 @@ func InvitationAddHintsAllowed(builder *flatbuffers.Builder, hintsAllowed bool)
 func InvitationAddHintsPerPlayer(builder *flatbuffers.Builder, hintsPerPlayer int32) {
 	builder.PrependInt32Slot(6, hintsPerPlayer, 0)
 }
+func InvitationAddMultipleWordsPerTurn(builder *flatbuffers.Builder, multipleWordsPerTurn bool) {
+	builder.PrependBoolSlot(7, multipleWordsPerTurn, false)
+}
 func InvitationAddDropoutTiles(builder *flatbuffers.Builder, dropoutTiles flatbuffers.UOffsetT) {
-	builder.PrependUOffsetTSlot(7, flatbuffers.UOffsetT(dropoutTiles), 0)
+	builder.PrependUOffsetTSlot(8, flatbuffers.UOffsetT(dropoutTiles), 0)
 }
 func InvitationAddStatus(builder *flatbuffers.Builder, status flatbuffers.UOffsetT) {
-	builder.PrependUOffsetTSlot(8, flatbuffers.UOffsetT(status), 0)
+	builder.PrependUOffsetTSlot(9, flatbuffers.UOffsetT(status), 0)
 }
 func InvitationAddGameId(builder *flatbuffers.Builder, gameId flatbuffers.UOffsetT) {
-	builder.PrependUOffsetTSlot(9, flatbuffers.UOffsetT(gameId), 0)
+	builder.PrependUOffsetTSlot(10, flatbuffers.UOffsetT(gameId), 0)
 }
 func InvitationAddExpiresAtUnix(builder *flatbuffers.Builder, expiresAtUnix int64) {
-	builder.PrependInt64Slot(10, expiresAtUnix, 0)
+	builder.PrependInt64Slot(11, expiresAtUnix, 0)
 }
 func InvitationEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
 	return builder.EndObject()