feat(bar-app): tune density and add spend chart style toggle

Narrow the dropdown (380 to 360) and give it more vertical room (620 to 700).
Tighten the subscription cards (less spacing/padding) since they carry only a
few window rows. Make the spend chart taller (18 to 30) and let the user pick
its style in Settings: chunk bars (default) or a line graph with a subtle area
fill. Persist the choice in UserDefaults.

macos-bar/Sources/CCSBarCore/BarChartStyle.swift

@@ -0,0 +1,35 @@
+import Foundation
+
+// MARK: - SpendChartStyle
+
+/// User-selectable render style for the spend sparkline in the dropdown.
+///
+/// `.bars` (default) draws the existing RoundedRectangle bar chart.
+/// `.line` draws a Path-based line graph with a faint area fill, which is
+/// better for spotting trend direction across the 30-day window.
+///
+/// Sendable + CaseIterable so the harness can iterate all cases and the value
+/// can cross actor boundaries safely.
+public enum SpendChartStyle: String, CaseIterable, Sendable {
+  case bars
+  case line
+}
+
+// MARK: - SpendChartStyleStore
+
+/// Persists the chosen spend-chart style. Mirrors the BarAppearanceStore pattern:
+/// a UserDefaults key, a static load, and a static save. The `?? .bars` fallback
+/// on a nil/unrecognized raw value is the sole source of the default.
+public enum SpendChartStyleStore {
+  public static let defaultsKey = "ccsbar.spendChartStyle"
+
+  public static func load() -> SpendChartStyle {
+    let raw = UserDefaults.standard.string(forKey: defaultsKey)
+      ?? SpendChartStyle.bars.rawValue
+    return SpendChartStyle(rawValue: raw) ?? .bars
+  }
+
+  public static func save(_ style: SpendChartStyle) {
+    UserDefaults.standard.set(style.rawValue, forKey: defaultsKey)
+  }
+}