fix(core,fuse): keep working through network drops

The mount only ever went from offline to online: nothing marked it
offline again, so after the Wi-Fi dropped every create, mkdir, delete and
rename called Drive, waited out the SDK's retries for up to two minutes
and answered EIO. Background loops kept polling the dead link.

Connectivity now lives in pdfs-fuse's new link module and goes both
ways. A call that gets no answer (a transport error or a request
timeout) marks the mount offline; a probe thread checks every 2 s,
backing off to 30 s, and on recovery re-lists folders served from the
database, retries failed operations and wakes the drain. Interactive
calls get a 20 s deadline, so a dead link costs one slow call instead of
many.

While offline:
- create, mkdir, unlink, rmdir and rename fall back to the offline
  queue instead of answering EIO;
- folders list from the database even when not marked listed, and are
  re-listed once the link returns;
- reads of uncached data wait up to 30 s for the link, then retry once
  within the 120 s read deadline;
- statfs serves the last quota instead of fetching, and quota refreshes
  are single-flight;
- the event and photo event loops wait for the link instead of polling.

A create whose request timed out may have landed anyway. When the drain
replays it and finds a same-named twin of the same kind, created within
the last two minutes and still empty, it adopts that node instead of
making a conflict copy. The drain also leaves alone a name still held
by a queued trash.

crates/pdfs-core/src/db/nodes.rs

@@ -559,6 +559,15 @@ impl Db {
         if listed != Some(1) {
             return Ok(None);
         }
+        drop(conn);
+        self.known_children(parent).map(Some)
+    }
+
+    /// The children the DB knows under `parent`, whether or not the folder's
+    /// listing is still marked complete. Only for when nothing better can be
+    /// had: a listing dropped as stale may miss or keep a child.
+    pub fn known_children(&self, parent: &NodeUid) -> Result<Vec<Node>> {
+        let conn = self.read();
         let mut stmt = conn.prepare(
             "SELECT node_json FROM nodes
              WHERE parent_uid = ?1 AND node_json IS NOT NULL AND trashed = 0",
@@ -568,7 +577,7 @@ impl Db {
         for json in rows {
             out.push(serde_json::from_str(&json?)?);
         }
-        Ok(Some(out))
+        Ok(out)
     }
 
     // --- Content-cache LRU index (P4) -------------------------------------