Stage 17 #2: Connecting indicator + auto-retry, instead of red toasts

Connectivity failures become state, not a toast on every attempt. A global online
signal (lib/connection.svelte.ts) flips on a transport unavailable / rate_limited and
on the live stream's drop, driving a pure-CSS header spinner + 'Connecting…' in place
of the title and softly disabling the in-game server actions (commit / exchange / pass
/ hint; local board/rack/reset stay live).

- transport: exec auto-retries with capped exponential backoff — every op on a
  rate-limit (rejected before processing, safe), reads only on unavailable (a mutation
  is never blindly re-sent, to avoid double-applying one whose response was lost; its
  button is disabled while offline so the player re-issues on reconnect). A reachability
  watcher (profile.get probe) and any successful traffic clear the signal.
- the old red error.unavailable toast is gone (handleError suppresses connection codes;
  the indicator replaces it). A server-data screen still opens with the spinner and
  fills on reconnect (global indicator + read auto-retry), so navigation is never dead.
- pure retry policy unit-tested (retry.ts); a mock-only window.__conn hook drives a
  Chromium+WebKit e2e (indicator shows offline, the action disables, both clear on
  reconnect). Full suite + build green.
- docs: ARCHITECTURE transport note, FUNCTIONAL (+ _ru), PLAN tracker (incl. #1 — the
  bot already drains all updates, no change).

Also records #1 as investigated/no-change in PLAN. Other server-action buttons (chat
send, profile save, …) still degrade to a safe no-op offline; visual disable is easy to
extend.

ui/src/game/Game.svelte

@@ -10,6 +10,7 @@
   import { gateway } from '../lib/gateway';
   import { navigate } from '../lib/router.svelte';
   import { app, handleError, showToast } from '../lib/app.svelte';
+  import { connection } from '../lib/connection.svelte';
   import { GatewayError } from '../lib/client';
   import { t } from '../lib/i18n/index.svelte';
   import type { Direction, EvalResult, MoveRecord, StateView, Tile } from '../lib/model';
@@ -720,7 +721,7 @@
         />
       </div>
       {#if !gameOver && placement.pending.length > 0}
-        <button class="make" onclick={commit} disabled={busy || !isMyTurn || (preview !== null && !preview.legal)} aria-label={t('game.makeMove')}>✅</button>
+        <button class="make" onclick={commit} disabled={busy || !isMyTurn || !connection.online || (preview !== null && !preview.legal)} aria-label={t('game.makeMove')}>✅</button>
       {/if}
     </div>
   {:else}
@@ -730,14 +731,14 @@
   {#snippet tabbar()}
     {#if view}
       <TabBar>
-        <button class="tab" disabled={busy || !isMyTurn || bagEmpty} onclick={openExchange}>
+        <button class="tab" disabled={busy || !isMyTurn || !connection.online || bagEmpty} onclick={openExchange}>
           <span class="sq">🔄</span><span class="lbl">{t('game.draw')}</span>
         </button>
-        <HoldConfirm triggerClass="tab" disabled={busy || !isMyTurn} onhold={doPass}>
+        <HoldConfirm triggerClass="tab" disabled={busy || !isMyTurn || !connection.online} onhold={doPass}>
           {#snippet trigger()}<span class="sq">🥺</span><span class="lbl">{t('game.skip')}</span>{/snippet}
           {#snippet popover(close)}<button class="pop go" onclick={() => { close(); doPass(); }}>{t('game.confirm')} ✅</button>{/snippet}
         </HoldConfirm>
-        <HoldConfirm triggerClass="tab" disabled={busy || !isMyTurn || (view?.hintsRemaining ?? 0) <= 0} onhold={doHint}>
+        <HoldConfirm triggerClass="tab" disabled={busy || !isMyTurn || !connection.online || (view?.hintsRemaining ?? 0) <= 0} onhold={doHint}>
           {#snippet trigger()}
             <span class="sq">🛟{#if (view?.hintsRemaining ?? 0) > 0}<span class="badge">{view?.hintsRemaining}</span>{/if}</span>
             <span class="lbl">{t('game.hint')}</span>