fix(menubar): cost budget stays USD; flag empty custom budget

mac/Sources/CodeBurnMenubar/AppStore.swift

@@ -85,9 +85,11 @@ final class AppStore {
         return total >= activeDailyBudget
     }
 
-    /// The active daily-budget threshold formatted for display (currency or tokens).
+    /// The active daily-budget threshold formatted for display (tokens, or USD).
+    /// The cost budget is defined in USD (matching the "$" presets and field), so
+    /// it is not run through the display-currency conversion here.
     var dailyBudgetLabel: String {
-        isTokenMetric ? "\(activeDailyBudget.asCompactTokens()) tokens" : activeDailyBudget.asCurrency()
+        isTokenMetric ? "\(activeDailyBudget.asCompactTokens()) tokens" : activeDailyBudget.asUSD()
     }
 
     var isLoading: Bool { loadingCountsByKey.values.contains { $0 > 0 } }
@@ -1268,6 +1270,12 @@ private let thousandsFormatter: NumberFormatter = {
         if n >= 1_000 { return String(format: "%.0fK", n / 1_000) }
         return String(format: "%.0f", n)
     }
+
+    /// Formats a raw USD amount with a "$" and grouping, without applying the
+    /// display-currency rate. Used for the USD-denominated daily budget.
+    func asUSD() -> String {
+        "$" + (groupedDecimalFormatter.string(from: NSNumber(value: self)) ?? "\(Int(self))")
+    }
 }
 
 extension Int {

mac/Sources/CodeBurnMenubar/Views/SettingsView.swift

@@ -57,6 +57,17 @@ private struct GeneralSettingsTab: View {
         v == v.rounded() ? String(Int(v)) : String(v)
     }
 
+    // Help text under the budget picker. When "Custom…" is selected but no amount
+    // has been entered, the budget is effectively 0 (off); call that out so the
+    // alert does not look armed when it isn't.
+    private var alertHelpText: String {
+        let customEmpty = store.isTokenMetric
+            ? (tokenCustom && store.dailyTokenBudget == 0)
+            : (costCustom && store.dailyBudget == 0)
+        if customEmpty { return "Enter an amount above, or the alert stays off." }
+        return "Flame icon turns yellow when today's \(store.isTokenMetric ? "tokens" : "cost") pass the daily budget."
+    }
+
     var body: some View {
         Form {
             Section("Display") {
@@ -162,7 +173,7 @@ private struct GeneralSettingsTab: View {
                         }
                     }
                 }
-                Text("Flame icon turns yellow when today's \(store.isTokenMetric ? "tokens" : "cost") pass the daily budget.")
+                Text(alertHelpText)
                     .font(.system(size: 11))
                     .foregroundStyle(.secondary)
             }