Implement a private/read-only mode

[C0de3nchantress]

Fixes #1203

Implement a private/read-only mode in Kiwix Desktop to protect user privacy by not writing anything to storage.

• Add a new method isPrivateMode() in src/settingsmanager.cpp and src/settingsmanager.h to check for the presence of a .private file.
• Modify src/kiwixapp.cpp and src/kiwixapp.h to include a check for private mode and prevent any write operations if the mode is active.
• Update src/mainwindow.cpp and src/mainwindow.h to add a toggle option in the UI for activating/deactivating private mode.
• Add documentation in README.md explaining the private/read-only mode and how to activate it via the UI or a .private file.

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For more details, open the Copilot Workspace session.