plugins: merge the user Plugins page into per-plugin sidebar pages

The generic per-user #plugins page is gone: a plugin with per-user
settings hosts them in its own web_pages() sidebar page instead
(Telegram's pairing page is new; Honcho's opt-in page already existed).
The admin catalog moves from #plugin-catalog to #plugins (old hash
redirected), and user_config_schema is removed from the Plugin trait,
the API DTOs and both plugins — the my-config endpoint, the
plugin_user_configs store and the update_user_config hook stay, now
driven by each plugin's own page fragment.

web/app.js

@@ -14,7 +14,6 @@ import { RolesPage }          from './components/roles-page.js';
 import { SharedFoldersPage }  from './components/shared-folders.js';
 import { ConnectorsPage }     from './components/connectors.js';
 import { ConnectorDetailPage } from './components/connector-detail.js';
-import { PluginsPage }        from './components/plugins-page.js';
 import { PluginPageHost }     from './components/plugin-page-host.js';
 import { PluginCatalogPage }  from './components/plugin-catalog.js';
 import { PluginDetailPage }   from './components/plugin-detail.js';
@@ -56,7 +55,6 @@ customElements.define('roles-page',           RolesPage);
 customElements.define('shared-folders-page',  SharedFoldersPage);
 customElements.define('connectors-page',      ConnectorsPage);
 customElements.define('connector-detail-page', ConnectorDetailPage);
-customElements.define('plugins-page',         PluginsPage);
 customElements.define('plugin-page-host',     PluginPageHost);
 customElements.define('plugin-catalog-page',  PluginCatalogPage);
 customElements.define('plugin-detail-page',   PluginDetailPage);