Preferences: prefs.set refuses a missing or unusable recovery or plug-ins folder

prefs.set did not check the folder given as recoveryFolder or pluginsFolder,
beyond the automation roots check, so a control or MCP caller could save a
folder that does not exist, a file, or a folder the app cannot use. Only the
Preferences dialog checked the folder itself.

The dialog's folder checks move to the engine, and prefs.set runs them after the
automation roots check. A folder that fails returns the same error the dialog
shows, and no preference changes. An empty value, an omitted value and a folder
the preference already holds are not checked. The Wasm build skips the check, as
the dialog did. The dialog's OK now gets the check from prefs.set, and Choose…
calls the moved check directly.

docs/plugins.md

@@ -52,7 +52,7 @@ background ([below](#heavy-filters)).
 **Preference folder.** Edit › Preferences › Plug-ins & Scratch Disks › *Additional Plug-ins Folder*: when set, every
 `*.wasm` in that folder (not recursive) is installed at startup and whenever the preference changes (desktop builds;
 the web build installs from bytes). On the desktop, Choose… beside the field picks the folder; the dialog refuses a
-picked or typed folder whose contents can't be listed.
+picked or typed folder whose contents can't be listed, and `prefs.set` refuses such a folder too.
 
 Plug-ins are installed **per process**, not per document. Documents store a live effect as an ordinary effect record
 (`{"id": "plugin.<id>", "params": {…}}`): while the plug-in is installed it is drawn, exported and expanded; without it