Ask the file, not its folder, why the write was refused

The 403 probe read `oc:permissions` off the parent collection and warned
when `W` was missing. But Nextcloud reports `W` on files and `CK` on
collections, so a directory legitimately lacks `W`: the warning fired on
a healthy share and pointed at a mount that was fine.

Probe the file itself. Its permissions answer the question that matters,
and the status distinguishes the two cases the parent could not: a 404
means the sidecar does not exist and the refusal was about creating it,
while a 200 without `W` means it exists and cannot be updated.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

core/dr-sync-nextcloud/src/lib.rs

@@ -373,16 +373,18 @@ impl RemoteBackend for NextcloudBackend {
             // make anything worse and it is the difference between a
             // server-side fix and a client-side one.
             if status == reqwest::StatusCode::FORBIDDEN {
-                let parent = path
-                    .as_str()
-                    .rsplit_once('/')
-                    .map(|(head, _)| head)
-                    .unwrap_or("");
+                // The *file*, not the folder. `W` is reported on files and
+                // `CK` on collections, so a directory legitimately lacks `W`
+                // and reading that as read-only would send someone to change a
+                // mount that is fine. If the file exists without `W` the
+                // refusal is an update it will not allow; a 404 here means it
+                // does not exist and the refusal was about creating it.
+                let target = path.as_str().to_string();
                 let probe = self
                     .client
                     .request(
                         reqwest::Method::from_bytes(b"PROPFIND").expect("valid method"),
-                        self.url_for(&RemotePath::new(parent)),
+                        self.url_for(path),
                     )
                     .basic_auth(&self.login, Some(&self.password))
                     .header("Depth", "0")
@@ -394,6 +396,7 @@ impl RemoteBackend for NextcloudBackend {
                     .await;
                 match probe {
                     Ok(r) => {
+                        let status = r.status();
                         let text = r.text().await.unwrap_or_default();
                         let perms = text
                             .split("<oc:permissions>")
@@ -401,11 +404,11 @@ impl RemoteBackend for NextcloudBackend {
                             .and_then(|t| t.split('<').next())
                             .unwrap_or("(not reported)");
                         log::warn!(
-                            "  parent {parent} permissions: {perms} \
-                             (W = write, C = create; absent means read-only)"
+                            "  target {target} -> {status} permissions: {perms} \
+                             (on a file W = update; a 404 means it does not exist yet)"
                         );
                     }
-                    Err(e) => log::warn!("  could not read parent permissions: {e}"),
+                    Err(e) => log::warn!("  could not read {target} permissions: {e}"),
                 }
             }