Backend infers play direction; UI previews words and gates submit on legality

A single tile that only extended a word perpendicular to the client-declared
direction was rejected: the UI always sent dir=H for one-tile plays (the
dirOverride/Controls toggle was orphaned in the Stage 7 game rework), so placing
"А" above "БАК" to form "АБАК" failed the solver's main-word-length check even
though the word is in the dictionary.

Make the backend infer a play's orientation from the placed tiles and the board
(internal/engine.resolveDirection): two or more tiles by the line they share, a
lone tile by the axis it abuts (longer word wins, horizontal on a tie). Direction
becomes an output, not an input: drop dir from the SubmitPlay/Eval wire requests
and add it to EvalResult. Journal replay keeps trusting the stored "H"/"V"
(SubmitPlayDir) so a rebuilt game matches the one committed.

UI: stop computing/sending direction; the preview now shows the words a move
forms with its total score (game.previewWords); the make-move control is disabled
until the play is confirmed legal; the "your turn" label hides while tiles are
pending. Delete the orphaned Controls.svelte.

Regenerate the FlatBuffers bindings (Go + TS) and update the gateway transcode
and the loadtest edge client to the new contract. Bake the decision into
ARCHITECTURE.md (§5/§9.1), FUNCTIONAL.md (+ _ru) and the backend README.

backend/internal/server/dto.go

@@ -1,8 +1,6 @@
 package server
 
 import (
-	"strings"
-
 	"scrabble/backend/internal/account"
 	"scrabble/backend/internal/engine"
 	"scrabble/backend/internal/game"
@@ -297,15 +295,3 @@ func chatDTOFrom(m social.Message) chatDTO {
 		CreatedAtUnix: m.CreatedAt.Unix(),
 	}
 }
-
-// parseDirection maps the wire direction string to an engine.Direction.
-func parseDirection(s string) (engine.Direction, bool) {
-	switch strings.ToUpper(strings.TrimSpace(s)) {
-	case "H":
-		return engine.Horizontal, true
-	case "V":
-		return engine.Vertical, true
-	default:
-		return 0, false
-	}
-}