feat(nudge): name the sender; blink lobby cards; re-animate toasts

The "waiting for your move" popup was the nudge (chat.nudge), shown without a
sender name. Resolve the nudger's per-game seat name server-side and carry it on
a new NudgeEvent.sender_name field, so:

- the in-app toast reads "<opponent>: Waiting for your move 🤭" (chat.nudgeBy);
- the out-of-app Telegram push names the sender too (render nudgeBy);

falling back to the plain phrase when the name is absent (RU mirrored). The
your_turn toast already named the opponent and is unchanged.

Lobby: when a card transitions into "your turn" or "finished" while the lobby is
open, its status emoji blinks twice (two 1s fades); the opponent's-turn change
stays in place. Blink state is keyed by game id (SvelteSet + per-id nonce/timer)
so overlapping events animate in isolation; suppressed under reduce-motion.

Toast: a per-message seq re-keys Toast.svelte, so the freshest toast cancels the
previous one and replays its entrance, uniformly on every screen.

Tests: notify.Nudge round-trip + render named/fallback (Go), game.Service.SeatName
(integration), codec/i18n/gamePhase/shouldBlink (UI). Docs (FUNCTIONAL +_ru,
UI_DESIGN) + PLAN TODO-7 (deferred FLIP card-relocation animation) updated.

backend/internal/social/chat.go

@@ -154,7 +154,10 @@ func (svc *Service) Nudge(ctx context.Context, gameID, senderID uuid.UUID) (Mess
 	}
 	svc.metrics.recordChat(ctx, kindNudge)
 	if toMove >= 0 && toMove < len(seats) {
-		nudge := notify.Nudge(seats[toMove], gameID, senderID)
+		// Name the sender by their per-game seat snapshot, so the toast and the out-of-app push
+		// read "<name>: …"; an unresolved name (best-effort) falls back to the plain phrase.
+		senderName, _ := svc.games.SeatName(ctx, gameID, senderID)
+		nudge := notify.Nudge(seats[toMove], gameID, senderID, senderName)
 		if lang, err := svc.games.GameLanguage(ctx, gameID); err == nil {
 			nudge.Language = lang // route by the game's bot, not the recipient's last-login one
 		}

backend/internal/social/social.go

@@ -25,6 +25,10 @@ import (
 // private state.
 type GameReader interface {
 	Participants(ctx context.Context, gameID uuid.UUID) (seats []uuid.UUID, toMove int, status string, err error)
+	// SeatName is the display name of the account seated in the game (its per-game snapshot,
+	// falling back to the account's current name), or "" when it holds no seat; it names a
+	// nudge's sender in the toast and the out-of-app push by the same identity the game uses.
+	SeatName(ctx context.Context, gameID, accountID uuid.UUID) (string, error)
 	// SharedGame reports whether two accounts are seated together in any game
 	// (active or finished); it gates the "befriend an opponent" request path.
 	SharedGame(ctx context.Context, a, b uuid.UUID) (bool, error)