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/CheckScreen.svelte

@@ -2,6 +2,8 @@
   import { onMount } from 'svelte';
   import { gateway } from '../lib/gateway';
   import { gameSource, isLocalGameId } from '../lib/gamesource';
+  import { connection } from '../lib/connection.svelte';
+  import { localWordCheck } from '../lib/dict/check';
   import { handleError, showToast } from '../lib/app.svelte';
   import { t } from '../lib/i18n/index.svelte';
   import { alphabetLetters } from '../lib/alphabet';
@@ -14,8 +16,12 @@
   let { id }: { id: string } = $props();
 
   let variant = $state<Variant>('scrabble_en');
+  let dictVersion = $state('');
   let word = $state('');
   let result = $state<{ word: string; legal: boolean } | null>(null);
+  // unavailable is set when an offline check cannot run because the game's pinned dictionary is not
+  // on the device (not cached/bundled); the panel then shows a neutral "offline" note.
+  let unavailable = $state(false);
   let cooling = $state(false);
   const checked = new Map<string, boolean>();
 
@@ -26,6 +32,7 @@
       // network.
       const st = await gameSource(id).gameState(id, true);
       variant = st.game.variant;
+      dictVersion = st.game.dictVersion;
     } catch (e) {
       handleError(e);
     }
@@ -33,6 +40,7 @@
 
   function onInput(e: Event) {
     word = sanitizeCheckWord((e.target as HTMLInputElement).value, alphabetLetters(variant));
+    unavailable = false;
   }
   // Disabled while cooling, for an already-checked word, or an out-of-range length.
   function canCheck(): boolean {
@@ -43,7 +51,22 @@
     const w = word.trim().toUpperCase();
     cooling = true;
     setTimeout(() => (cooling = false), 5000);
+    unavailable = false;
     try {
+      // Offline in an online game the network check_word would fail, so fall back to the game's own
+      // pinned dictionary read on-device: exact when that dawg is cached/bundled, otherwise the check
+      // is unavailable. A local game already resolves checkWord on-device, so it keeps that path.
+      if (!isLocalGameId(id) && !connection.online) {
+        const legal = await localWordCheck(variant, dictVersion, w);
+        if (legal === null) {
+          result = null;
+          unavailable = true;
+          return;
+        }
+        checked.set(w, legal);
+        result = { word: w, legal };
+        return;
+      }
       const r = await gameSource(id).checkWord(id, w, variant);
       checked.set(w, r.legal);
       result = { word: w, legal: r.legal };
@@ -73,7 +96,9 @@
     />
     <button onclick={runCheck} disabled={!canCheck()}>{t('game.check')}</button>
   </div>
-  {#if result}
+  {#if unavailable}
+    <p class="verdict">{t('game.checkOffline')}</p>
+  {:else if result}
     <p class="verdict" class:ok={result.legal} class:bad={!result.legal}>
       {result.legal
         ? t('game.wordLegal', { word: result.word })
@@ -82,10 +107,10 @@
     <div class="actions">
       <!-- Complaints go to the admin over the network; a local (offline) game has no backend to
            receive them, so the control is dropped there. -->
-      {#if !isLocalGameId(id)}
+      {#if !isLocalGameId(id) && connection.online}
         <button class="complain" onclick={complain}>{t('game.complain')}</button>
       {/if}
-      {#if result.legal}
+      {#if result.legal && connection.online}
         <a
           class="lookup"
           href={dictionaryLookupUrl(result.word, variant)}