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/lib/app.svelte.ts

@@ -23,6 +23,8 @@ import {
 } from './telegram';
 import { parseStartParam } from './deeplink';
 import { clearSession, loadPrefs, loadSession, saveSession, savePrefs } from './session';
+import { reportOffline, reportOnline, resetConnection } from './connection.svelte';
+import { isConnectionCode } from './retry';
 import { clearGameCache } from './gamecache';
 import { clearLobby } from './lobbycache';
 import type { BoardLabelMode } from './boardlabels';
@@ -121,6 +123,9 @@ export function handleError(err: unknown): void {
       void logout();
       return;
     }
+    // A connectivity failure is shown by the "Connecting…" header indicator (and auto-retried),
+    // not a red toast on every attempt.
+    if (isConnectionCode(err.code)) return;
     showToast(t(errorKey(err.code)), 'error');
     return;
   }
@@ -132,6 +137,7 @@ function openStream(): void {
   streamAlive = true;
   unsubscribeStream = gateway.subscribe(
     (e) => {
+      reportOnline(); // a delivered event proves the gateway is reachable
       app.lastEvent = e;
       if (e.kind === 'chat_message' && e.message.senderId !== app.session?.userId) {
         // While the player is on that game's chat screen, neither toast nor bump the unread.
@@ -155,9 +161,10 @@ function openStream(): void {
     },
     () => {
       streamAlive = false;
-      // A background suspend drops the single-shot stream. Keep the banner hidden while
-      // backgrounded or just-resumed (bannerSuppressed); always schedule a retry.
-      if (!bannerSuppressed()) showToast(t('error.unavailable'), 'error');
+      // A background suspend drops the single-shot stream. Keep the indicator hidden while
+      // backgrounded or just-resumed (bannerSuppressed); otherwise show "Connecting…" (the
+      // reachability watcher and this scheduled retry recover it). Always schedule a retry.
+      if (!bannerSuppressed()) reportOffline();
       scheduleReconnect();
     },
   );
@@ -399,6 +406,7 @@ export async function loginEmail(email: string, code: string): Promise<void> {
 
 export async function logout(): Promise<void> {
   closeStream();
+  resetConnection();
   clearGameCache();
   clearLobby();
   gateway.setToken(null);