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>

README.md

@@ -73,7 +73,7 @@ The interface is available in English and German; ink-notes follows the system l
 - Every file is first written completely to a temporary file and then replaced – a crash while saving never leaves a half-written file.
 - **When closing** (window or `Ctrl + Q`) and when switching notebooks, everything pending is written before continuing. If that fails, the app stays open and offers to retry or to make an emergency backup.
 - `Ctrl + S` saves immediately. The dot on the ☰ button shows the state: green = saved, orange = changes pending, yellow = saving, red = error.
-- **Storage location** (☰ menu, "Storage location"): put the notebooks into a folder that **Nextcloud, Google Drive, Syncthing or Dropbox** keep in sync, and they are available on all your devices. The notebooks are kept in the subfolder `ink-notes`; if it is still empty, the existing notebooks are copied there (the old ones stay). Changes arriving from another device show up within a few seconds without reopening – but only while everything local is saved, so nothing gets overwritten. If the sync client makes a conflict copy of a page, it is added as a page of its own („Conflict copy: …“) instead of getting lost. Opening a notebook that is open on another device shows a warning.
+- **Storage location** (☰ menu, "Storage location"): put the notebooks into a folder that **Nextcloud, Google Drive, Syncthing or Dropbox** keep in sync, and they are available on all your devices. The notebooks are kept in the subfolder `ink-notes`; if it is still empty, the existing notebooks are copied there (the old ones stay). Changes arriving from another device show up within a few seconds without reopening – but only while everything local is saved, so nothing gets overwritten. If the sync client makes a conflict copy of a page, it is added as a page of its own („Conflict copy: …“) instead of getting lost. The same happens when a change arrives just while you are writing on that page: your version is saved and theirs is kept as a conflict copy. Opening a notebook that is open on another device shows a warning.
 - **Nextcloud built in** (☰ menu, "Nextcloud"; also on Android and without a sync client): enter the address of your Nextcloud – also a private server, in a subfolder (`example.org/nextcloud`) or with its own port. Signing in happens in the browser (also with two-factor sign-in); the app gets an app password of its own, which you can revoke in Nextcloud under Settings → Security at any time, and never sees your real password. The notebooks are synced with the folder `ink-notes` in your Nextcloud files every 30 seconds and shortly after saving; the app keeps working offline. If a page was changed on two devices, both versions are kept (the other one as a page of its own); deleted notebooks and pages go into the trash (locally `.trash`, on the server its trash bin). A self-signed certificate is trusted once you have confirmed its fingerprint; addresses with `http://` only work with a warning, since the app password would be readable on the network. "Disconnect" stops syncing and revokes the app password; the notebooks stay. Use either the built-in sync or a sync client's folder as storage location, not both for the same notebooks.
 
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