Add library_stats aggregate query for the sidebar summary

New LibraryStats (unique cards owned, total copies, deck count, and
per-color copy counts including colorless) via Catalog::library_stats,
tested against the fixture catalog.

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

crates/mtgc-core/src/model.rs

@@ -139,6 +139,22 @@ pub struct Deck {
     pub notes: String,
 }
 
+/// Aggregate stats about the owned library, for a dashboard-style summary.
+#[derive(Debug, Clone, Default, Serialize, Deserialize)]
+pub struct LibraryStats {
+    /// Distinct oracle cards owned (quantity_owned > 0).
+    pub unique_cards: i64,
+    /// Total physical copies owned, summed across all cards.
+    pub total_copies: i64,
+    pub deck_count: i64,
+    /// Copies owned in each color, WUBRG order. A multicolor card's copies
+    /// count toward every color it contains, so these can sum to more than
+    /// `total_copies`.
+    pub color_counts: [i64; 5],
+    /// Copies owned with no color (colors == 0).
+    pub colorless_count: i64,
+}
+
 /// Criteria for searching the owned library. All fields are optional; an empty
 /// filter matches every owned card.
 #[derive(Debug, Clone, Default, Serialize, Deserialize)]