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>

crates/mtgc-core/src/error.rs

@@ -16,6 +16,9 @@ pub enum CoreError {
     #[error("json error: {0}")]
     Json(#[from] serde_json::Error),
 
+    #[error("cancelled")]
+    Cancelled,
+
     #[error("{0}")]
     Other(String),
 }