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/i18n/en.js

@@ -21,7 +21,6 @@ export default {
   'nav.roles':         'Roles',
   'nav.connectors':    'Connectors',
   'nav.plugins':       'Plugins',
-  'nav.plugin_catalog': 'Plugin Catalog',
   'nav.config':        'Settings',
   'nav.llm_requests':  'LLM Requests',
   'nav.system_agents': 'System agents',
@@ -874,39 +873,25 @@ export default {
   'connectors.error.no_connector':    'No connector named "{name}" is available to you.',
 
   // ── Plugins ─────────────────────────────────────────────────────────────────
-  'plugins.title':             'Plugins',
   'plugins.loading':           'Loading…',
-  'plugins.section.mine':      'My plugins',
-  'plugins.section.manage':    'Manage plugins',
-  'plugins.empty.mine':        'No plugins available to you yet.',
-  'plugins.empty.ask_admin':   'Ask an admin to grant you access.',
   'plugins.empty.manage':      'No plugins registered.',
-  'plugins.status.active':     'active',
-  'plugins.status.running':    'running',
-  'plugins.status.enabled':    'enabled',
-  'plugins.status.off':        'off',
   'plugins.enabled':           'Enabled',
-  'plugins.save':              'Save',
   'plugins.save_config':       'Save config',
   'plugins.saved':             'Saved.',
   'plugin_page.loading':       'Loading…',
   'plugin_page.unavailable':   'This page is not available (plugin disabled or page not granted).',
-  'plugins.yes':               'yes',
-  'plugins.no':                'no',
-  'plugins.badge.user_config': 'per-user settings',
-  'plugins.access.btn':        'User access',
+  'plugins.badge.user_page':   'user page',
   'plugins.access.desc':       'Tick a box to let that person see and configure this plugin. Saving replaces the whole list.',
   'plugins.access.empty':      'No users.',
   'plugins.access.save':       'Save access',
   'plugins.error.required':    '"{field}" is required.',
-  'plugins.catalog.title':     'Plugin Catalog',
   'plugins.catalog.configure': 'Configure',
   'plugins.health.ok':           'active',
   'plugins.health.off':          'off',
   'plugins.health.needs_config': 'needs configuration',
   'plugins.health.not_running':  'not running',
   'plugins.badge.instance_config': 'instance settings',
-  'plugins.detail.back':         'Back to catalog',
+  'plugins.detail.back':         'Back to plugins',
   'plugins.detail.config.title': 'Instance configuration',
   'plugins.detail.config.empty': 'This plugin has no instance settings.',
   'plugins.detail.config.custom_page': 'This plugin has its own configuration page.',