Overhaul card-data ingest: filter art cards, segmented cancellable progress

- Exclude Scryfall Art Series cards (layout "art_series") from ingest —
  they aren't playable cards and were polluting search results.
- Replace the coarse per-stage progress text with a byte-accurate,
  weighted multi-segment progress bar (Index / Card data / Printings /
  Indexing) that fills as one continuous bar instead of swapping per
  stage; segment weights are computed from the real download sizes
  reported by Scryfall's bulk-data index (the download responses
  themselves are chunked with no Content-Length to rely on).
- Add a Cancel button, wired through a new CancelToken checked once per
  parsed record so cancellation lands almost instantly.
- Fix a related correctness bug this surfaced: ingest used to delete the
  existing reference.db before writing its replacement, so a cancelled
  or crashed update left the app with no card database at all. It now
  builds into a temp file and only atomically replaces reference.db on
  full success.
- Split the topbar's single "Choose library…" action into separate
  "Open Library…" (existing file) and "New Library…" (save-dialog for a
  new file) buttons.

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

src-tauri/src/commands.rs

@@ -50,22 +50,39 @@ pub fn set_library_path(db: Db, path: String) -> Res<Status> {
 #[tauri::command]
 pub async fn update_card_data(app: AppHandle, db: Db<'_>) -> Res<Status> {
     let reference_path = db.lock().unwrap().reference_path.clone();
+    let cancel = mtgc_core::ingest::CancelToken::new();
+    db.lock().unwrap().ingest_cancel = Some(cancel.clone());
     let app_for_progress = app.clone();
 
-    tauri::async_runtime::spawn_blocking(move || {
-        mtgc_core::ingest::build_reference_db(&reference_path, |msg| {
-            let _ = app_for_progress.emit("ingest-progress", msg.to_string());
-        })
+    let result = tauri::async_runtime::spawn_blocking({
+        let cancel = cancel.clone();
+        move || {
+            mtgc_core::ingest::build_reference_db(&reference_path, &cancel, |p| {
+                let _ = app_for_progress.emit("ingest-progress", p);
+            })
+        }
     })
     .await
-    .map_err(|e| e.to_string())?
     .map_err(|e| e.to_string())?;
 
+    db.lock().unwrap().ingest_cancel = None;
+    result.map_err(|e| e.to_string())?;
+
     let mut s = db.lock().unwrap();
     s.reopen()?;
     Ok(status_of(&s))
 }
 
+/// Cancel an in-progress `update_card_data`, if one is running. No-op otherwise.
+#[tauri::command]
+pub fn cancel_update_card_data(db: Db) -> Res<()> {
+    let s = db.lock().unwrap();
+    if let Some(c) = &s.ingest_cancel {
+        c.cancel();
+    }
+    Ok(())
+}
+
 #[tauri::command]
 pub fn search_cards(db: Db, query: String, limit: usize) -> Res<Vec<CardRef>> {
     let s = db.lock().unwrap();