Add card USD pricing and a deck maybeboard

Prices come from Scryfall's prices.usd on printings, shown as a hover badge
on card tiles. The price is picked independently of whichever printing
supplies the display image (the cheapest known price across all printings),
since the display printing is often a promo/variant with no listed price.

The maybeboard lets a deck track cards under consideration without counting
them in deck analysis stats or library assignment totals. Cards move between
mainboard and maybeboard via a per-card toggle; maybeboarding the commander
clears commander status.

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

crates/mtgc-core/src/model.rs

@@ -99,6 +99,9 @@ pub struct CardRef {
     /// `produced_mana` including "C"), tracked separately since colorless
     /// isn't a WUBRG bit.
     pub produces_colorless: bool,
+    /// USD market price (Scryfall's `prices.usd`) for the same printing that
+    /// supplied `image_uri`, or 0 if unknown.
+    pub price_usd: f64,
 }
 
 /// A library row: a card the user owns, with quantity and its deck memberships.
@@ -125,6 +128,10 @@ pub struct DeckEntry {
     /// Copies of this card in the deck.
     pub quantity: i64,
     pub is_commander: bool,
+    /// On the maybeboard: tracked against the deck for consideration, but
+    /// excluded from deck analysis stats and library assignment totals —
+    /// it isn't actually part of the deck.
+    pub is_maybeboard: bool,
     /// Copies owned in the library (0 if the card isn't in the library).
     pub quantity_owned: i64,
 }