fix: handle file overwrite in completion installation

Fix completion installation failing when completion files already exist.
The installCompletionFile method now properly removes existing files
after backup creation, allowing copyItem to succeed.

Fixes issue where `usbipd completion install` would fail with:
- "usbipd" couldn't be copied because an item with the same name already exists
- "_usbipd" couldn't be copied because an item with the same name already exists
- "usbipd.fish" couldn't be copied because an item with the same name already exists

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

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

Author: beriberikix

Sources/USBIPDCore/CLI/CompletionInstaller.swift

@@ -419,6 +419,11 @@ public class CompletionInstaller {
     ///   - targetPath: Target installation path
     /// - Throws: Installation errors
     private func installCompletionFile(from sourcePath: String, to targetPath: String) throws {
+        // Remove existing file if it exists (backup was already created in the calling method)
+        if FileManager.default.fileExists(atPath: targetPath) {
+            try FileManager.default.removeItem(atPath: targetPath)
+        }
+        
         try FileManager.default.copyItem(atPath: sourcePath, toPath: targetPath)
 
         // Set appropriate permissions