"feat : add incognito mode"

[Fatir227]

Description

This pull request introduces Incognito Mode to Nyxt Browser.
When enabled, Incognito Mode ensures that:

• No browsing history is recorded.
• No cookies or session data are stored.
• A clear indicator is shown to the user that they are browsing privately.

This feature provides users with a privacy-focused browsing option similar to other modern browsers.

Fixes #247

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Changes Made

• Added incognito-mode command to start a private browsing session.
• Disabled history persistence while incognito is active.
• Prevented cookies/session data from being stored in incognito buffers.
• Added a UI indicator in the mode-line to reflect incognito status.

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Checklist

• Branch is mergeable and up-to-date with master.
• No new dependencies introduced.
• Documentation updated where necessary.
• Feature tested locally and working as expected.

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Usage

1. Launch Nyxt.
2. Run the command: