feat(ui): explicit offline state inside an online game (+ offline word check)

When an online game loses the connection the game screen now says so and freezes
instead of silently greying out: a "connection lost" banner appears and the rack,
the move controls, the add-friend/block controls and the chat/dictionary entry all
disable (a started move stays a draft, committed by the player on reconnect). It is
driven off the net-state machine (netState.offline), so it also covers the
Telegram/VK mini-apps, where a lost connection was previously mute in-game.

If the player is already in the dictionary when the drop happens, the word check
falls back to the game's pinned on-device dictionary (exact when that dawg is
cached; "unavailable offline" otherwise) and the network-only complaint + external
look-up hide. Chat, when already open, keeps its existing read-only degrade
(send/nudge disable). New pure helper localWordCheck is unit-tested; the in-game
gating gets an e2e.

ui/src/game/Rack.svelte

@@ -13,6 +13,7 @@
     shuffling = false,
     draggingId = null,
     dropIndex = null,
+    frozen = false,
     confirm,
     ondown,
   }: {
@@ -26,6 +27,10 @@
     // the drag ghost stands in) and dropIndex is the slot where a gap opens.
     draggingId?: number | null;
     dropIndex?: number | null;
+    /** frozen disables tile interaction (offline in an online game): tiles cannot be picked up to
+     *  compose or reorder a move until the connection returns. The confirm control is gated
+     *  separately by the parent. */
+    frozen?: boolean;
     /** The confirm-move control, rendered as the fixed 7th slot of the rack while a play is staged
      *  (the parent owns the button and its enablement; the rack only positions it). */
     confirm?: Snippet;
@@ -65,6 +70,7 @@
       class:selected={selected === slot.index}
       class:shift={dropIndex != null && i >= dropIndex}
       data-rack-index={slot.index}
+      disabled={frozen}
       animate:hop={shuffling}
       onpointerdown={(e) => ondown(e, slot.index)}
     >
@@ -114,6 +120,10 @@
     outline: 3px solid var(--accent);
     outline-offset: -3px;
   }
+  /* Frozen (offline in an online game): the tray is inert until the connection returns. */
+  .tile:disabled {
+    opacity: 0.5;
+  }
   /* While reordering, tiles at/after the drop slot slide right to open a gap there (one
      tile width plus the rack gap), so the drop position is visible. */
   .rack.reordering .tile {