Remove the duplicate window controls

[qaid]

Problem

The floating detail panel has two close controls:

1. Native macOS close button (red traffic-light): Added by .closable in the NSPanel style mask (Sources/Spook/App/AppDelegate.swift, DetailPanel.init(), line ~164).
2. Custom SwiftUI close button (xmark.circle.fill): Rendered in the DetailPanelContentView header (line ~225).

Both buttons close the panel. The native close button appears in the transparent titlebar area above the custom SwiftUI header, creating a confusing and visually cluttered experience that is inconsistent with the panel's custom HUD-style design.

Fix

Remove .closable from the NSPanel style mask:

// Before (AppDelegate.swift ~line 164):
styleMask: [.titled, .closable, .fullSizeContentView, .nonactivatingPanel],

// After:
styleMask: [.titled, .fullSizeContentView, .nonactivatingPanel],

The custom xmark.circle.fill button in DetailPanelContentView is the intended close control and must remain unchanged.

Acceptance Criteria

• The native red traffic-light close button no longer appears on the floating panel.
• The custom SwiftUI close button (xmark.circle.fill) in the panel header still closes the panel correctly.
• The pin button still toggles correctly.
• The panel remains draggable by its background area (isMovableByWindowBackground = true).
• No other window behaviour changes (floating level, spaces behaviour, shadow, transparency).

Relevant File

• Sources/Spook/App/AppDelegate.swift, DetailPanel.init() — the styleMask parameter on line ~164.