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.

ui/src/lib/i18n/en.ts

@@ -225,6 +225,7 @@ export const en = {
   'friends.block': 'Block',
   'friends.add': 'Add a friend',
   'friends.addFromGame': 'Add to friends',
+  'friends.blockFromGame': 'Block player',
   'friends.requestSent': 'Friend request sent.',
   'friends.getCode': 'Show my code',
   'friends.codeHint': 'Give this code to a friend within 12 hours.',
@@ -284,6 +285,7 @@ export const en = {
   'game.exportGcg': 'Export GCG',
   'game.gcgActiveOnly': 'Available once the game is finished.',
   'game.addFriendShort': 'Add friend?',
+  'game.blockShort': 'Block?',
 
   'time.minutes': '{n} min',
   'time.hours': '{n} h',