Add location filter to library search

LibraryFilter.location does a case-insensitive substring match against
the stored location (e.g. "Box 1"), mirroring the existing text filter.
Tested for substring/case-insensitivity and the empty-string no-op case.

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

crates/mtgc-core/src/model.rs

@@ -181,4 +181,7 @@ pub struct LibraryFilter {
     pub keywords: Vec<String>,
     /// Restrict to cards assigned to this deck id.
     pub in_deck: Option<i64>,
+    /// Case-insensitive substring match against the card's stored location
+    /// (e.g. "Box 1"). Cards with no location set never match a non-empty value.
+    pub location: Option<String>,
 }