fix(fuse): keep pinned files on disk through remote changes

A remote change to a pinned file called `ContentCache::evict`, which does
not check pins, and only the pin request itself ever downloaded anything.
An edit from another device, or even a rename, left the file listed as kept
offline with no copy, so the next read downloaded it or failed without a
network. A file added to a pinned folder elsewhere was never downloaded,
and a folder pin whose downloads failed while offline never retried them.

A remote change no longer evicts a pinned file's copy. The copy is checked
against the revision's (mtime, size), so a newer revision does not read
it, and a rename keeps the mtime (B123), so the copy stays valid. A new
pdfs-pin-keeper thread checks pins again and downloads files that are
missing or stale:

- the pins a batch of remote events changed, after the batch's listings
  are dropped, including a pinned folder that gets a new file;
- every pin at startup, when the link returns, and after the event feed
  loses continuity;
- a file whose version is restored.

Failed downloads retry with a backoff of 30 s to 30 min. Pinning and the
keeper settle a provisional size first: a copy stored under the
ciphertext size never matched the plaintext it holds.

Fixes B282.

docs/CHANGELOG.md

@@ -59,6 +59,13 @@ Existing folders stay device backups. Older builds cannot open the upgraded data
   Proton Drive mount, so the copy in the synced folder was downloaded again by every program that
   read it.
 
+- **Files kept offline stay on this computer when they change elsewhere** (B282). A change from
+  another device, even a rename, deleted the copy of a file kept offline, and nothing downloaded
+  it again, so the next read downloaded it or failed without a network. Files added to a folder
+  kept offline from another device were never downloaded. The copy now stays until the new
+  version is downloaded, which happens on its own. Files kept offline are also checked at
+  startup and when the connection returns, and failed downloads are tried again.
+
 ## [3.7.0] — 2026-11-02
 
 Schema: **44**, unchanged.