fix(ui): match settings height to list mode + CI uses latest Xcode

UI fix (SDK-agnostic):
- SettingsView height 480 → 620 so the settings mode panel size matches
  list mode at maximum. Avoids MenuBarExtra's NSPanel not shrinking when
  switching between list and settings views.
- Drop the .id(showSettings) hack — it forced view rebuild but the panel
  is managed by AppKit and does not respond to SwiftUI view recreation.

CI hardening:
- Use maxim-lobanov/setup-xcode@v1 with latest-stable so the runner Xcode
  is as close to local (Xcode 26.5) as available, minimizing SwiftUI SDK
  behavior drift.
- Run xcodebuild test before building the DMG; release will fail-fast on
  test regressions instead of shipping broken binaries.

Root cause analysis of v1.3.4 regression:
Local Xcode 26.5 produced a 5.2MB binary linking to macOS 26 SwiftUI
symbols; GH macos-latest with default Xcode 16 produced a 5.0MB binary
linking to macOS 15 SwiftUI. The .id() hack happened to look correct in
the SDK 26 build but not in SDK 15 — same source, different runtime
behavior. The height:620 approach works in both.

StockMonitor/Views/DropdownView.swift

@@ -105,6 +105,5 @@ struct DropdownView: View {
         .frame(width: 320)
         .fixedSize(horizontal: false, vertical: true)
         .background(Color(nsColor: .windowBackgroundColor))
-        .id(showSettings)
     }
 }

StockMonitor/Views/SettingsView.swift

@@ -258,7 +258,9 @@ struct SettingsView: View {
             .padding(8)
         }
         .scrollIndicators(.never)
-        .frame(width: 284, height: 480)
+        // 高度与列表模式（ProfitSummary + stockListView maxHeight 520 + Toolbar 等）相当，
+        // 避免切换 list↔settings 时 NSPanel 大小不一致导致下方空白。
+        .frame(width: 284, height: 620)
     }
 
     // MARK: - 排序辅助