Persist last-opened library file across restarts; fix path-switch bug

- Remember which library.db was last chosen (via a small config.json in
  the app-data dir) so relaunching the app reopens the same file instead
  of always defaulting to library.db.
- Fix set_library_path returning a hard error (and thus never updating
  the topbar) whenever the reference catalog fails to reopen against the
  new path — e.g. a stale reference.db schema. Picking a library file
  and successfully reattaching the reference db are separate concerns;
  the chosen path now always persists and is reported back, with the
  catalog reopened best-effort.

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

src-tauri/src/commands.rs

@@ -40,8 +40,12 @@ pub fn get_status(db: Db) -> Res<Status> {
 #[tauri::command]
 pub fn set_library_path(db: Db, path: String) -> Res<Status> {
     let mut s = db.lock().unwrap();
-    s.library_path = PathBuf::from(path);
-    s.reopen()?;
+    s.set_library_path(PathBuf::from(path));
+    // Best-effort reopen: the chosen path must stick (and be reported back)
+    // even if the reference db can't currently be attached (e.g. stale
+    // schema pending an "Update card data" run) — that's a separate concern
+    // from which library file is open.
+    let _ = s.reopen();
     Ok(status_of(&s))
 }
 

src-tauri/src/state.rs

@@ -1,11 +1,37 @@
 //! App state: the open [`Catalog`] and fuzzy [`NameIndex`], plus the db paths.
 
-use std::path::PathBuf;
+use std::path::{Path, PathBuf};
 
 use mtgc_core::{Catalog, NameIndex};
+use serde::{Deserialize, Serialize};
+
+/// Small app-shell preferences persisted across restarts (distinct from the
+/// user's library data) — currently just which library file was last open.
+#[derive(Default, Serialize, Deserialize)]
+struct PersistedConfig {
+    library_path: Option<PathBuf>,
+}
+
+fn config_path(data_dir: &Path) -> PathBuf {
+    data_dir.join("config.json")
+}
+
+fn load_config(data_dir: &Path) -> PersistedConfig {
+    std::fs::read_to_string(config_path(data_dir))
+        .ok()
+        .and_then(|s| serde_json::from_str(&s).ok())
+        .unwrap_or_default()
+}
+
+fn save_config(data_dir: &Path, cfg: &PersistedConfig) {
+    if let Ok(s) = serde_json::to_string_pretty(cfg) {
+        let _ = std::fs::write(config_path(data_dir), s);
+    }
+}
 
 /// Managed application state, guarded by a `Mutex` in Tauri's state store.
 pub struct AppState {
+    data_dir: PathBuf,
     pub library_path: PathBuf,
     pub reference_path: PathBuf,
     pub catalog: Option<Catalog>,
@@ -16,12 +42,18 @@ pub struct AppState {
 }
 
 impl AppState {
-    /// Initialize with default paths under the app data dir and open the catalog
-    /// if a reference db already exists.
+    /// Initialize with the last-opened library path (or the default under the
+    /// app data dir, on first run) and open the catalog if a reference db
+    /// already exists.
     pub fn new(data_dir: PathBuf) -> Self {
+        let cfg = load_config(&data_dir);
+        let library_path = cfg
+            .library_path
+            .unwrap_or_else(|| data_dir.join("library.db"));
         let mut s = Self {
-            library_path: data_dir.join("library.db"),
             reference_path: data_dir.join("reference.db"),
+            library_path,
+            data_dir,
             catalog: None,
             index: None,
             ingest_cancel: None,
@@ -31,6 +63,18 @@ impl AppState {
         s
     }
 
+    /// Point at a different library file and persist it so the next launch
+    /// reopens the same one.
+    pub fn set_library_path(&mut self, path: PathBuf) {
+        self.library_path = path.clone();
+        save_config(
+            &self.data_dir,
+            &PersistedConfig {
+                library_path: Some(path),
+            },
+        );
+    }
+
     /// (Re)open the catalog + rebuild the name index. Only opens when a reference
     /// db exists — attaching a non-existent file would create an empty (tableless)
     /// db and make every query fail.