Fix WKWebView crash on tab close

[Acumen-Desktop]

Fix WKWebView crash on tab close

Problem

When closing a WKWebView tab, the app crashes with signal 11 (SIGSEGV).

Root Cause

1. Dangling pointer: The remove method was releasing the webview and then accessing it in an async dispatch_async block, causing a use-after-free
2. KVO observer leak: The fullscreenState KVO observer was only removed in dealloc, but by then self.webView was already nil

Solution

• Remove KVO observer first, before any cleanup
• Remove webview from superview synchronously instead of async dispatch
• Add safety checks in dealloc for already-removed observers

Testing

Tested by repeatedly opening and closing both CEF and WebKit tabs - no more crashes.