Add DEBUG-gated panel to buy any card in the shop

When the server's DEBUG env var is on, expose a card catalog endpoint
and a collapsible client panel that drops any pet or food straight into
your deck (free, off-turn). Gated server-side so it is inert in normal
play.

internal/game/view.go

@@ -35,6 +35,9 @@ type View struct {
 	Pending    *PendingTrade `json:"pending,omitempty"`
 	Battle     *BattleResult `json:"battle,omitempty"`
 	WinnerSeat int           `json:"winnerSeat"`
+	// Debug is set by the server when its DEBUG flag is on, unlocking the
+	// client's "buy any card" panel. Not part of the pure game state.
+	Debug bool `json:"debug,omitempty"`
 }
 
 // ViewFor builds the state visible to the given player.