refactor(web): restructure into api/domain/services/worker; capabilities, file domains, video covers

- dissolve core-bridge into api (wire contract), domain (models), services (what the UI
  calls) and worker (everything that runs in the Web Worker, grouped by account, crypto,
  space, files, shares, runtime); regroup features, lib and store by responsibility and
  delete unused components
- the UI only reads backend capabilities (domain/capabilities); one tokenRole picks the
  token, hasCode/isRetryable handle errors; pre-checks that decided permissions on the
  client are gone; tests fail if those rules are reimplemented
- files go straight to the file domain (create, upload, download, delete, commit, cancel);
  the host sees one registration per file; creating a channel allocates a file domain and
  works even when the file service is down
- file panel shows which channel the files belong to and its space usage; unlimited files
  preview in chat; videos get a stored cover; the file service status is shown
- fix: console flooded by Retry-After reads on every upload frame

web/src/features/account/AccountScreen.tsx

@@ -0,0 +1,12 @@
+import { AccountPanel } from './AccountPanel';
+import { ThemeMenu } from '@/ui/ThemeMenu';
+import styles from '@/features/home/Home.module.css';
+export function AccountScreen() {
+  return <div className={styles.page}><div className={styles.appearance}><ThemeMenu /></div><main className={`${styles.main} ${styles.authMain}`}>
+    <header className={styles.authIntro}>
+      <h1 className={styles.authLogo}>WebDrop</h1>
+      <p>登录后，可在其他设备恢复本账号保存的空间和分享。</p>
+    </header>
+    <AccountPanel variant="auth" />
+  </main></div>;
+}