fix(storage): keep a page changed by another device when saving over it

A change from another device could reach the page file while a stroke was
being drawn; the check that takes it over waits until everything is saved,
so the next autosave overwrote it. save_page now notices that the file is
not what this process last read or wrote: their version is kept as a
conflict copy (which becomes a page of its own), or simply stays if there
is nothing new to save here.

Co-Authored-By: Claude Fable 5.2 <noreply@anthropic.com>

docs/FILE_FORMAT.md

@@ -177,6 +177,7 @@ If the notebooks are kept in a folder synced by Nextcloud, Google Drive, Syncthi
 - `.opened_by.json` (`{"device": "<host name>", "since": <unix time>}`) is written when a notebook is opened and removed when it is closed. A marker from another device leads to a warning; the lock file `.lock` only protects against a second instance on the same computer.
 - While all local changes are saved, the app checks the folder every 3 seconds (and when the window gets the focus). Changed page files are reloaded, a changed `manifest.json` brings in new, removed or reordered pages and titles.
 - Conflict copies of sync clients (`page_<id> (….json`, `page_<id>_….json`, `page_<id>.sync-conflict-….json`) are added as new pages after the original and renamed to `<name>.imported`.
+- Before the app overwrites a page file that changed since it last read or wrote it, it keeps the content found there as `page_<id> (conflict <unix time>).json`, unless it has no changes of its own (then the file is left as it is).
 
 ### Newer versions
 Files with a higher `format_version` than supported are not opened – this way an older app version cannot lose content it does not know.