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.

docs/UI_DESIGN.md

@@ -131,10 +131,14 @@ except Login uses `Screen`.
   (badged with unread chat) at the right, icon-only. A **single-word-rule** game (a Russian
   game with "multiple words per turn" off) centres a **"One word per turn"** label between
   those two icons, and the status bar shows a small **1️⃣** in the score-preview slot that
-  yields to the live word/score preview while tiles are pending; standard games show neither. Each **opponent**'s card also gains a
-  🤝 **add-friend** control (non-guests; hidden once a friend, disabled once requested) that
-  confirms via the fading-✅ tap, swapping the card's score for "Add friend?" while armed
-  (see Controls); the name and score stay centred — the 🤝 is pinned to the card's edge.
+  yields to the live word/score preview while tiles are pending; standard games show neither. Each **opponent**'s card also gains two
+  edge-pinned controls (non-guests): a 🤝 **add-friend** on the right (hidden once a friend,
+  disabled once requested) and a ✖️ **block** on the left. Each confirms via the fading-✅ tap,
+  swapping the card's score for "Add friend?" — or **"Block?" in the danger colour** — while
+  armed (see Controls); while one is armed the other hides so they never overlap. Once the
+  opponent is **blocked** both controls go and the **name is struck through** (and the comms-hub
+  chat composer is hidden — only the log remains). The name and score stay centred — the controls
+  are pinned to the card's edges.
   Unread chat is also badged on the score bar itself, so it shows with the history closed.
 - **Vertical fit & keyboard**: when the game does not fit the viewport, only the
   board area scrolls vertically (`Screen` `column` mode; the score bar, status, rack and tab
@@ -180,8 +184,8 @@ except Login uses `Screen`.
   tap-to-confirm control. A first tap arms a ~2 s window showing a **fading ✅** (no fade
   under reduce-motion, but the window still holds); a tap on the ✅ within it confirms,
   otherwise it reverts. Used by the **Hint** tab (the icon morphs to ✅, no label — replacing
-  the old press-and-hold popover) and the in-game **add-friend 🤝**. The
-  **Drop game** action keeps its `Modal` confirmation (a destructive, less-frequent action).
+  the old press-and-hold popover) and the in-game **add-friend 🤝** and **block ✖️** card
+  controls. The **Drop game** action keeps its `Modal` confirmation (a destructive, less-frequent action).
 - **MakeMove / Reset**: when ≥1 tile is pending the rack collapses its used slots
   and shifts left, a **borderless ✅ icon button** (styled like a tab, not a filled accent
   button) beside the rack commits the move — no popover, and disabled while the pending word