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_test.go

@@ -14,30 +14,6 @@ import (
 	"scrabble/backend/internal/social"
 )
 
-func TestParseDirection(t *testing.T) {
-	cases := map[string]struct {
-		in   string
-		want engine.Direction
-		ok   bool
-	}{
-		"horizontal":      {"H", engine.Horizontal, true},
-		"vertical":        {"V", engine.Vertical, true},
-		"lowercase":       {"h", engine.Horizontal, true},
-		"trimmed":         {" V ", engine.Vertical, true},
-		"invalid":         {"X", 0, false},
-		"empty":           {"", 0, false},
-		"diagonal-is-not": {"D", 0, false},
-	}
-	for name, tc := range cases {
-		t.Run(name, func(t *testing.T) {
-			got, ok := parseDirection(tc.in)
-			if ok != tc.ok || (ok && got != tc.want) {
-				t.Fatalf("parseDirection(%q) = (%v, %v), want (%v, %v)", tc.in, got, ok, tc.want, tc.ok)
-			}
-		})
-	}
-}
-
 func TestStatusForError(t *testing.T) {
 	cases := map[string]struct {
 		err        error