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/game/Chat.svelte

@@ -13,6 +13,7 @@
     waiting = false,
     nudgeOnCooldown = false,
     vsAi = false,
+    blocked = false,
     onsend,
     onnudge,
   }: {
@@ -38,6 +39,10 @@
     // vsAi disables both controls in an honest-AI game: the opponent is a robot, so there is
     // nobody to message or hurry. The fields still show (turn-driven), but disabled.
     vsAi?: boolean;
+    // blocked hides the whole composer (message field, send and nudge), leaving only the chat
+    // log — as in a finished game — when the viewer has blocked the opponent: there is no one
+    // they will message (the backend rejects it too).
+    blocked?: boolean;
     onsend: (text: string) => void;
     onnudge: () => void;
   } = $props();
@@ -65,6 +70,7 @@
       {/if}
     {/each}
   </div>
+  {#if !blocked}
   <div class="input">
     {#if canSend}
       <input
@@ -85,6 +91,7 @@
       <button class="iconbtn" onclick={onnudge} disabled={busy || waiting || nudgeOnCooldown || vsAi || !connection.online} aria-label={t('chat.nudgeAction')}>🛎️</button>
     {/if}
   </div>
+  {/if}
 </div>
 
 <style>