Fix code completion window visibility issues

Resolves intermittent visibility problems where the code completion window would not appear visually despite functioning correctly for keyboard navigation.

Changes:
- Replace problematic window style masks (.nonactivatingPanel, .utilityWindow) with .borderless
- Set hidesOnDeactivate to false to prevent unexpected hiding
- Add canHide = false and acceptsMouseMovedEvents = true for better window behavior
- Improve window showing sequence in showWindow(attachedTo:)
- Add proper cleanup of notification observers in close()

The .nonactivatingPanel style was preventing the window from becoming key, causing the "canBecomeKeyWindow returned NO" warning and inconsistent visibility. Using .borderless provides the clean appearance needed for code completion while ensuring reliable display.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: lucasmotta

Sources/CodeEditSourceEditor/CodeSuggestion/Window/SuggestionController+Window.swift

@@ -84,7 +84,7 @@ extension SuggestionController {
     static func makeWindow() -> NSWindow {
         let window = NSWindow(
             contentRect: .zero,
-            styleMask: [.resizable, .fullSizeContentView, .nonactivatingPanel, .utilityWindow],
+            styleMask: [.borderless],
             backing: .buffered,
             defer: false
         )
@@ -97,8 +97,10 @@ extension SuggestionController {
         window.hasShadow = true
         window.isOpaque = false
         window.tabbingMode = .disallowed
-        window.hidesOnDeactivate = true
+        window.hidesOnDeactivate = false
         window.backgroundColor = .clear
+        window.canHide = false
+        window.acceptsMouseMovedEvents = true
 
         return window
     }

Sources/CodeEditSourceEditor/CodeSuggestion/Window/SuggestionController.swift

@@ -103,7 +103,10 @@ public final class SuggestionController: NSWindowController {
     /// Opens the window as a child of another window.
     public func showWindow(attachedTo parentWindow: NSWindow) {
         guard let window = window else { return }
+
+        super.showWindow(nil)
         parentWindow.addChildWindow(window, ordered: .above)
+        window.orderFront(nil)
 
         // Close on window switch observer
         // Initialized outside of `setupEventMonitors` in order to grab the parent window
@@ -118,8 +121,6 @@ public final class SuggestionController: NSWindowController {
             self?.close()
         }
 
-        super.showWindow(nil)
-        window.orderFront(nil)
         window.contentViewController?.viewWillAppear()
     }
 
@@ -134,6 +135,12 @@ public final class SuggestionController: NSWindowController {
             contentViewController?.viewWillDisappear()
         }
 
+        // Clean up window observers
+        if let observer = windowResignObserver {
+            NotificationCenter.default.removeObserver(observer)
+            windowResignObserver = nil
+        }
+
         super.close()
     }