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/state.rs

@@ -10,6 +10,9 @@ pub struct AppState {
     pub reference_path: PathBuf,
     pub catalog: Option<Catalog>,
     pub index: Option<NameIndex>,
+    /// Set while `update_card_data` is running; `cancel_update_card_data`
+    /// flips it to abort the in-progress ingest.
+    pub ingest_cancel: Option<mtgc_core::ingest::CancelToken>,
 }
 
 impl AppState {
@@ -21,6 +24,7 @@ impl AppState {
             reference_path: data_dir.join("reference.db"),
             catalog: None,
             index: None,
+            ingest_cancel: None,
         };
         // Best-effort open; failures just leave the catalog unloaded.
         let _ = s.reopen();