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.

crates/core-api/src/user_plugin_config.rs

@@ -6,7 +6,7 @@ use serde_json::Value;
 /// `system.db`).
 ///
 /// Values are deliberately admin-readable — the table lives in the registry
-/// database — so `user_config_schema`s must never collect secrets. A plugin
+/// database — so per-user plugin configs must never collect secrets. A plugin
 /// that needs per-user secrets should keep them elsewhere.
 #[async_trait]
 pub trait PluginUserConfigApi: Send + Sync {