fix: separate rounded navigation panel and adjust settings dialog styling

[Rello]

Motivation

• Remove the single rounded panel wrapping both navigation and content so the left navigation can remain a distinct rounded panel while content panels keep their own styling.
• Make the active navigation highlight narrower and rounded to match updated visual guidance.

Description

• Updated toolbar CSS in TOOLBAR_CSS() to use transparent toolbar background and to style QToolButton:checked with rounded corners and reduced margin for a narrower active indicator.
• Added a dedicated settings_navigation container (navigationContainer) with a QVBoxLayout and moved the _toolBar into it so the navigation can have its own rounded background.
• Adjusted the main shell layout to insert the new navigation container and updated the dialog stylesheet to set #settings_shell to transparent and #settings_navigation to a rounded background.
• Changes made in src/gui/settingsdialog.cpp (layout insertion and stylesheet updates).

Testing

• No automated tests were run for this UI/layout change.

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Codex Task