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/whisper_local.md

@@ -23,7 +23,7 @@ The model (roughly 1–3 GB depending on size) is loaded into memory only when f
 ## Enabling & configuring (admin)
 
 1. Download a model file first (see above) and note its path.
-2. Plugin catalog → **Whisper Local** → enable, then **Configure**.
+2. Plugins page → **Whisper Local** → enable, then **Configure**.
 3. Fields:
    - **`model`** (required) — path to the `.bin` file, e.g. `models/ggml-large-v3.bin`.
    - **`language`** — a BCP-47 code (`it`, `en`, …) or `auto` for automatic detection (default `auto`).