feat(social): asymmetric per-user block, in-game block control, admin lists

Make a per-user block one-directional and non-destructive: the blocker stops
receiving everything from the blocked user (chat, nudge, friend requests,
invitations) and the matchmaker never pairs them, while the blocked user
notices nothing — their sends still persist by the normal rules but are never
delivered or surfaced (born-read). A block no longer deletes the friendship
(an unblock cleanly restores it) and instant-reads any unread the blocked user
had left for the blocker.

- backend: a directional blockExists guard across chat/nudge/friends/invitations
  (store-but-hide for the blocked->blocker direction, refuse blocker->blocked);
  the matchmaker excludes a block-related pair (both directions) from auto-match;
  user_blocked/user_unblocked notifications to the blocker only (in-app only).
- ui: the opponent score card gains a block ✖️ control mirroring add-friend
  (red "Block?" confirm, mutual-hide while confirming, struck name, hidden chat
  composer when blocked); optimistic apply + event confirm + rollback for both.
- admin: the user card gains cross-linked blocks / blocked-by / friends lists.
- docs: FUNCTIONAL(+ru), ARCHITECTURE §10 + decision record, UI_DESIGN, PRERELEASE.

backend/internal/social/social.go

@@ -80,6 +80,10 @@ var (
 	ErrMessageNotFound = errors.New("social: message not found")
 	// ErrChatBlocked is returned when the sender has disabled chat for themselves.
 	ErrChatBlocked = errors.New("social: chat is disabled for this account")
+	// ErrRecipientBlocked is returned when a player tries to chat to, or nudge, someone
+	// they have blocked — the blocker-side guard behind the hidden composer (a blocked
+	// player's own sends are never refused, only silently suppressed, so they never notice).
+	ErrRecipientBlocked = errors.New("social: cannot message a blocked player")
 	// ErrMessageTooLong is returned when a chat message exceeds the rune limit.
 	ErrMessageTooLong = errors.New("social: message exceeds the length limit")
 	// ErrEmptyMessage is returned when a chat message is blank after trimming.