Merge: answer Android's memory warnings, and stop reporting a lost root as an empty library

FR-PLAT-AND-5 in full, FR-PLAT-AND-2 in part -- the recovery is built and
live for Nextcloud roots, the SAF cause it names does not exist yet.

FR-PLAT-AND-4 and FR-PLAT-AND-6 are not here, both blocked behind the
same gap: assemble-apk.sh compiles no Java, so the APK cannot carry a
Service or a FileProvider. The container has JDK 17 and build-tools 36;
the build step is what is missing.

Verified: fmt, clippy --workspace --all-targets -D warnings, and 1043
tests across dr-catalog, dr-sync, dr-sync-folder, dr-sync-nextcloud,
dr-plat and dr-ui. The aarch64 target was checked before the branch was
finished but not after; no device was available.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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

@@ -216,10 +216,17 @@ impl std::fmt::Debug for FolderBackend {
 impl FolderBackend {
     /// Open the folder at `root`.
     ///
-    /// The directory must exist now. It may stop existing later — a drive
-    /// unplugged, a mount dropped — and that surfaces per-operation as
-    /// [`RemoteError::Network`], which is what puts the app into offline mode
-    /// and leaves the catalog readable, exactly as a dead server does.
+    /// The directory must exist now, and not existing is
+    /// [`RemoteError::RootUnavailable`] — the library folder could not be
+    /// opened, which is the whole of what this knows. A drive unplugged
+    /// between sessions and a path typed wrongly at setup are the same
+    /// observation from here, and both are answered the same way: keep the
+    /// catalog, say which folder, and offer it again (FR-PLAT-AND-2).
+    ///
+    /// A mount dropped *during* a session surfaces per-operation as
+    /// [`RemoteError::Network`] instead, which is what puts the app into
+    /// offline mode and leaves the catalog readable, exactly as a dead server
+    /// does.
     pub fn new(root: impl Into<PathBuf>) -> Result<Self, RemoteError> {
         Self::with_vfs(root, Arc::new(NoVfs))
     }
@@ -232,7 +239,15 @@ impl FolderBackend {
     pub fn with_vfs(root: impl Into<PathBuf>, vfs: Arc<dyn Vfs>) -> Result<Self, RemoteError> {
         let root = root.into();
         if !root.is_dir() {
-            return Err(RemoteError::Configuration(format!(
+            // TRACES: FR-PLAT-AND-2
+            // Not `Configuration`, which is where this lived while there was
+            // nothing better. The distinction that matters is not "was the
+            // account written wrongly" — which nothing here can know — but
+            // "can this library be opened", and a caller that knows the
+            // library was working yesterday can act on the second answer:
+            // mark what it holds as offline rather than deleting it, and ask
+            // for the folder again (FR-CAT-9).
+            return Err(RemoteError::RootUnavailable(format!(
                 "{} is not a folder",
                 root.display()
             )));