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.

docs/plugins/remote_connectivity.md

@@ -19,7 +19,7 @@ Depends on which provider is chosen:
 
 ## Enabling & configuring (admin)
 
-1. Plugin catalog → **Remote Connectivity** → enable, then **Configure**.
+1. Plugins page → **Remote Connectivity** → enable, then **Configure**.
 2. Fields:
    - **`provider`** (`tailscale_sys` | `tailscale`, default `tailscale_sys`) — see requirements above.
    - **`auth_key`** — only for the embedded `tailscale` provider; a Tailscale auth key (`tskey-auth-…`), needed on first join.