android: what the phone says when things go wrong

Fifty-nine places put a raw Rust string into a field a person reads, and
the red toast drew it verbatim — a phone in a lift showed "could not reach
server: io: failed to lookup address information" under the password box.
One translator, src/mobile/trouble.js, called where an error is DRAWN
rather than where it is caught: the stores keep what Rust said and only
the sentence on screen is ours, so every sink is fixed at once. Five
screens that drew a failed read as an empty answer stop lying — Tracks no
longer reports a broken database as an empty library, a playlist that
could not be read is not an empty playlist, the lyrics face says a lookup
failed instead of "no lyrics" and finally carries the ⋯ that reaches Paste
lyrics, Gaps says why it could not reach the catalogue, and Search signed
out says so where it used to render nothing at all. The room list, which
is fire-and-forget over the wire, gets a twelve-second end and a Try
again.

rescan_library ran prune_missing() first, which deletes every track row
whose file does not exist — so one tap on You → Rescan library with the
folder unreachable emptied the whole index and returned Ok(()). It now
makes the root if it is merely not there yet and refuses if it cannot be
read, and You draws the path with what to do about it.

The mock grew a weather system so the walk can drive all of this: fail,
answer, offline and clear, checked before dispatch so a plugin command
arms like any other. Nine mock steps and one on the phone — the first
steps in thirty rounds that ever made a command reject or saw the client
signed out — and the last of them puts the weather away and leaves the app
as it found it.

src-tauri/src/pipeline.rs

@@ -423,12 +423,21 @@ fn downloads_dir(app: &tauri::AppHandle) -> PathBuf {
 /// until the next app launch.
 #[tauri::command]
 pub async fn rescan_library(app: tauri::AppHandle) -> Result<(), String> {
+    // The prune below deletes every track row whose file does not exist, and
+    // then every album left with none — so a root that has gone away (an
+    // unmounted card, a revoked SAF grant, a drive letter that moved) used to
+    // empty the WHOLE index on one tap and return Ok(()). Make the folder if
+    // it is merely not there yet, so a fresh install still works, and refuse
+    // if it cannot be made or read. Same idiom as connect_and_login's.
+    let root = downloads_dir(&app);
+    if std::fs::create_dir_all(&root).is_err() || !root.is_dir() {
+        return Err(format!("library-folder-missing: {}", root.display()));
+    }
     let state = app.state::<LibraryState>();
     let lib = state.library.clone();
     let _ = tauri::async_runtime::spawn_blocking(move || lib.prune_missing())
         .await
         .map_err(|e| e.to_string())?;
-    let root = downloads_dir(&app);
     sweep_stale_partials(&root);
     sweep_stale_auditions(&root);
     sweep_empty_dirs(&root, 0);