M1: save files automatically

Preferences > Save files automatically (pref.autoSave, off by default;
hidden in the browser, which has no files to save to). With it on, each
open file that has a path is saved 3 s after it changes, never during a
gesture, a live text preview or while a version is shown. A failed save
says why in a toast and tries again a minute later.

Version history checkpoints (version.autosave, every 30 minutes) only ran
for files with unsaved changes, so a file that saves itself would stop
getting them; with auto-save on, edits made since the file was opened
count.

Closes #296

docs/mcp.md

@@ -349,6 +349,8 @@ on; `key` resolves keys the way the canvas does. Commands behind keys:
   nested layers), Escape or a click drops it.
 - `pref.reduceMotion {on?}`: reduced motion (the interface's own animations off; the system's
   accessibility setting turns it on too). Prototype transitions still play.
+- `pref.autoSave {on?}`: files that have a path save themselves a few seconds after they change
+  (off by default; the desktop app's timer does it, so scripts still call `file.save`).
 - `pref.language {language: system|en|es|es-419|fr|de|pt-BR|ja|ko|zh-Hans|…}`: the app's interface language, applied live ("system":
   the OS or browser language; a language without a catalog shows English). Command ids, labels and
   every result stay English.