fix(app): guard NSApp access in error badge rendering

NSApp is nil in test/CI environment. Use optional chaining to
fall back to black (light mode) when unavailable.

Author: pasrom

app/MeetingTranscriber/Sources/MenuBarIcon.swift

@@ -97,8 +97,8 @@ enum MenuBarIcon {
 
             case .error:
                 // Use menu bar foreground color since this badge is non-template
-                let appearance = NSApp.effectiveAppearance
-                let isDark = appearance.bestMatch(from: [.darkAqua, .aqua]) == .darkAqua
+                let isDark = NSApp?.effectiveAppearance
+                    .bestMatch(from: [.darkAqua, .aqua]) == .darkAqua
                 (isDark ? NSColor.white : NSColor.black).setFill()
                 drawRecordingAnimation(in: rect, frame: 0)
                 drawExclamationBadge(in: rect)