Stability and Memory Safety Improvements

[niutech]

This submission addresses several critical and non-critical issues in the Split Browser codebase:

1. Crash Prevention: Added null-pointer checks for currentWebView() in MainWindow's slots. Previously, actions like toggling fullscreen or adding a bookmark would crash the application if no tabs were open.

2. Memory Leak Fixes:

   • COM/Win32: Fixed a leak in win32_edge_engine where LPWSTR strings from WebView2 were not freed with CoTaskMemFree.
   • Heap Management: Changed get_title() and get_url() to return class-managed strings instead of leaking new char[].
   • JS Bindings: Implemented a destructor in the webview class to clean up its bindings map. Added a safety mechanism to correctly identify and delete sync_binding_ctx_t pointers while avoiding user-provided pointers.
   • Static Managers: Added MainWindow::cleanup() to properly delete the static HistoryManager and BookmarksManager instances upon application exit.

3. Code Quality & UI:

   • URL Handling: Refactored sanitizeUrl to use QUrl::fromUserInput, making it significantly more robust than the previous hardcoded TLD-based logic.
   • Responsiveness: Removed a 500ms blocking sleep in clearData() that was unnecessarily freezing the UI thread.
   • Readability: Replaced magic numbers in event filters with standard Qt enum constants (e.g., QEvent::FocusIn).
   • I18n: Wrapped hardcoded user-facing strings in tr() and adopted the recommended .arg() pattern for variable interpolation in translations.

These changes collectively improve the stability, performance, and maintainability of the application.

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PR created automatically by Jules for task 7958240601656317970 started by @niutech

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