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/bin/ingest.rs

@@ -5,6 +5,8 @@
 use std::path::PathBuf;
 use std::process::ExitCode;
 
+use mtgc_core::ingest::CancelToken;
+
 fn main() -> ExitCode {
     let dest = std::env::args()
         .nth(1)
@@ -12,7 +14,10 @@ fn main() -> ExitCode {
         .unwrap_or_else(|| PathBuf::from("reference.db"));
 
     eprintln!("Building reference db at {}", dest.display());
-    match mtgc_core::ingest::build_reference_db(&dest, |msg| eprintln!("  {msg}")) {
+    let cancel = CancelToken::new();
+    match mtgc_core::ingest::build_reference_db(&dest, &cancel, |p| {
+        eprintln!("  [{}] {:.1}% {}", p.stage, p.overall_percent, p.message)
+    }) {
         Ok(stats) => {
             eprintln!(
                 "Done: {} oracle cards, {} printings.",