fix(linux): input not working, context menu locking focus, black screen when running in compose-hot-reload and white bg for websites with dark background

[KRTirtho]

Fixed bugs:

• WebKit2GTK input not working when using with Nucleus 2.5.18 while Copy/Paste (Ctrl+c and Ctrl+v etc) works fine
• Context menu locks focus and makes other Compose composables non-focusable. Replaced with GTK popup API.
• When running with compose-hot-reload, a black screen takes the place of entire webview although webview is running fine.
  Basically there is not proper output. Did a dirty workaround by painting the frames directly as Image. Makes rendering slightly slower and laggy but good enough for compose-hot-reload.
• A forced CSS snippet in the C code was causing certain websites with dark backgrounds #000 to have white backgrounds in Compose light mode. Making the website unusable.

New inclusions:

• A minimal demo desktop application
• Added a new visual test case (C12) to ensure opaque WebViews correctly preserve dark page backgrounds, and implemented the corresponding test logic.