Fix dark theme contrast and window close bug

[schlunsen]

Summary

• Fix window close bug: Window could not be closed after opening the Stats panel because NSApp.keyWindow was nil when the window lost key status. Now falls back to NSApp.mainWindow.
• Fix text contrast on dark backgrounds: Replaced all system .secondary color references with explicit Color.white.opacity(X) values in GlobeWindowView and StatsView, since the app uses custom hardcoded dark backgrounds that make adaptive system colors nearly invisible.
• Replace segmented pickers with custom buttons: Both the bottom tab bar and the Stats breakdown period picker now use custom styled buttons with proper white-on-dark contrast instead of the system .segmented picker style.

Changes

• GlobeWindowView.swift: Close button fallback, tab bar custom buttons, all .secondary → .white.opacity()
• StatsView.swift: Period picker custom buttons, all .secondary → .white.opacity(), added missing text colors

Test plan

• Open the Globe window and verify the Servers/Processes/History/Stats tab buttons are clearly visible
• Click the Stats tab, open the details panel, and verify the close button (red circle) still works
• Verify Weekly/Monthly toggle in the Breakdown section is readable
• Verify all text labels (stat cards, period rows, section headers) are visible on the dark background

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