Reader home tells new users to click a channel while the channel list is empty

Problem/Motivation

ReaderController::home() falls back to a static message when the current user has no default_timeline stored in user.data:

// src/Controller/ReaderController.php
$settings = $this->userData->get('reader', $this->currentUser()->id(), 'settings');
$default_timeline = !empty($settings['default_timeline']) ? $settings['default_timeline'] : '';
if (!empty($default_timeline)) {
  [$type, $id] = explode(ReaderInterface::SEPARATOR, $default_timeline);
  return $this->timeline($request, $type, $id, TRUE);
}

$build['main'] = ['#markup' => '<div class="general-content">' . $this->t('Click on a channel to start reading') . '</div>'];

On a fresh site with a fresh user, that instruction cannot be followed, because the channel list sitting right next to it is empty. With activitypub_reader enabled, `Reader::getChannels()` returns early when the user has no ActivityPub actor:

// modules/activitypub_reader/src/Reader.php
public function getChannels() {
  if (!$this->getActor()) {
    return [];
  }
  // ... Home / Notifications / Direct messages / Local timeline / Bookmarks
}

A newly registered user does not have an actor — creating one is an explicit opt-in at `/user//activitypub`, gated behind `allow users to enable activitypub`. The base `reader` module contributes a channel only when `aggregator` is installed.

So the two columns say contradictory things:

• main column — “Click on a channel to start reading”
• channels block — “No channels found”

The third column does offer Sources and Settings links, but both lead to empty
pages for this user, so the loop is closed:

• `/reader/settings` — the *Default timeline* select contains only `- None -`, because ReaderSettingsUserForm` builds its options from the same `hook_reader_channels` results.
• `/reader/sources` — lists nothing, because `ReaderController::sources()` skips any module whose channels are empty:

  foreach (reader_get_implementators() as $module) {
    $function = $module . '_reader_channels';
    if (function_exists($function)) {
      $data = $function();
      if (!empty($data['channels'])) {   // <-- skipped when the user has no actor
        $sources[] = Link::fromTextAndUrl(...);
      }
    }
  }

Every affordance visible to a new user leads back to an empty page, and none of them mentions the thing that would actually unblock them: creating an ActivityPub actor at /user/<uid>/activitypub.

It is worth separating two distinct states, because they need different answers.
They map one-to-one onto the two branches proposed further down.

1. The user has an actor, but never picked a default timeline. Five channels are available (`home`, `notifications`, `direct`, `local`, `bookmarks`). Today: one pointless extra click before they see anything.
2. The user has no actor — which is every newly registered user. Zero channels are available. Today: the dead end described above.

State 2 is the first thing anyone sees after the module is installed.

There is a second-order effect worth mentioning: because default_timeline lives in user.data rather than in config, a site owner cannot pre-seed it — not through config, not through a recipe, not through config:set. Recipes only reach configuration, and this is per-user runtime data. So “ship a site that works out of the box” is currently not achievable for Reader without custom code. (I worked around it in my own environment with a small drush php:script.) That makes the fallback below more valuable than it first looks: it is the only lever that does not require per-user provisioning.

Steps to reproduce

1. Install Drupal 11 with `reader`, `activitypub`, `activitypub_reader`, and install the `reader_theme` theme.
2. Grant `access reader` to the authenticated role.
3. Create a new user. Do not create an ActivityPub actor for them, and do not visit `/user//reader`.
4. Log in as that user and go to `/reader`.

Result:

column 1 : Click on a channel to start reading
column 2 : Read | No channels found
column 3 : Write | Blog post | Other | Posts | Sources | Settings

5. Follow the two links that look like a way out:

• `/reader/settings` → *Default timeline* select offers only `- None -`
• `/reader/sources` → empty list

For contrast, a user who *does* have an actor but still has no `default_timeline`
sees the same “Click on a channel to start reading”, while the channel column
correctly lists five channels — one click away from content:

Fediverse
  Home (1)
  Notifications
  Direct messages
  Local timeline
  Bookmarks

Proposed resolution

Remaining tasks

User interface changes

- Better guidance on /reader when either no actor is available, or when no modules are installed at all that integrate with reader
- ability to set default global timeline
- ability in code to override the #empty text - activitypub overrides this in case the global default timeline is set to activitypub_reader::home to create an actor when necessary.