feat(web): offline/reconnect/realname top banners (README §3.7)

Four states, one shown at a time (offline > reconnecting > just-reconnected
> realname-reminder priority): no-network (navigator.onLine + online/offline
listeners), reconnecting-with-countdown, just-reconnected, and a skippable
realname-verification reminder (bootstrap.security.realname).

- connection.ts: claims Tinode's onAutoreconnectIteration(timeoutMs, promise?)
  callback (single-slot, same pattern as onConnect/onDisconnect) and runs its
  own 1s-tick countdown off the first call's timeout -- the SDK gives the
  backoff length once per iteration, it doesn't push a running countdown.
  Verified the callback's exact three-shape contract (timeoutMs alone starts
  a countdown; 0+promise means a retry is now in flight; -1 means retrying
  stopped) by reading the SDK's own Connection class, not guessed.
  retryReconnectNow() backs "立即重试": clears the countdown and calls
  tinode.reconnect(true).
- "已重新连接，正在同步 N 条新消息" intentionally shown without a fake N --
  confirmed via research that the old app never implemented this state at
  all, and there's no reliable signal here for how many messages arrived
  while disconnected; inventing a count would be dishonest data.

Verified live: toggling browser online/offline events correctly shows and
clears the no-network banner.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/lib/api/bootstrap.ts

@@ -6,9 +6,17 @@ export interface MessagePolicy {
   recall_window_seconds?: number;
 }
 
+export interface RealnameStatus {
+  required: boolean;
+  allow_skip: boolean;
+  remind_after?: string;
+  status: 'unverified' | 'pending' | 'verified' | string;
+}
+
 export interface BootstrapConfig {
   message_policy?: MessagePolicy;
   limits?: { max_forward_targets?: number; [key: string]: unknown };
+  security?: { realname?: RealnameStatus };
   [key: string]: unknown;
 }