Convert im_webapp from static mock prototypes to real backend integration

Implements all 11 product chapters (auth, messaging, contacts, groups,
files/favorites, calls, workplace/reports, settings, security) against the
real /v1/im REST API and native Tinode WS protocol, replacing the hardcoded
mock data the ba8f48ee prototype commit shipped with. Adds src/lib/im-api.js
as the REST client, plus supporting fixes across messages-view.jsx and the
attachment/upload/download pipeline (wrong SDK auth headers, group topic
naming, seq/timezone edge cases) found via live end-to-end testing against
the real backend.

Backup checkpoint before reconciling with origin/dev's parallel rewrite.

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

src/widgets/new-topic-group.jsx

@@ -9,6 +9,7 @@ import TagManager from './tag-manager.jsx';
 import { AVATAR_SIZE, MAX_AVATAR_BYTES, MAX_EXTERN_ATTACHMENT_SIZE, MAX_TITLE_LENGTH,
   MAX_TOPIC_DESCRIPTION_LENGTH } from '../config.js';
 import { imageScaled, blobToBase64, makeImageUrl } from '../lib/blob-helpers.js';
+import { createChatUploader } from '../lib/im-api.js';
 import { truncateString } from '../lib/strformat.js';
 
 export default class NewTopicGroup extends React.PureComponent {
@@ -69,7 +70,9 @@ export default class NewTopicGroup extends React.PureComponent {
       let {mime, blob} = image;
       if (blob.size > MAX_AVATAR_BYTES) {
         // Too large to send inband - uploading out of band and sending as a link.
-        const uploader = this.props.tinode.getLargeFileHelper();
+        // Same wrong-header bug messages-view.jsx's chat uploads had -- createChatUploader() is the
+        // fixed-headers drop-in (see im-api.js for the full explanation).
+        const uploader = createChatUploader(this.props.tinode);
         uploader.upload(blob)
           .then(url => this.setState({imageUrl: url}))
           .catch(err => this.props.onError(err.message, 'err'));