refactor: replace e2e notice with widget layout

[Rello]

Motivation

• The E2E notice previously used a KMessageWidget with a colored background which made it visually inconsistent with other account settings controls.
• The goal is to make the E2E notice look like the rest of the Account Settings panel by using a normal QWidget layout with icon, text and actions.
• Remove the previous toggle-based styling change to KMessageWidget and instead drive the notice presentation directly from the Account Settings UI.

Description

• Replaced the KMessageWidget instance in src/gui/accountsettings.ui with a plain QWidget containing an icon QLabel, a text QLabel and a horizontal layout for action buttons.
• Implemented UI-driving logic in src/gui/accountsettings.cpp to set the notice text via _ui->encryptionMessageLabel, set the icon via setEncryptionMessageIcon(), and rebuild action buttons with updateEncryptionMessageActions() using QToolButtons wired to QActions.
• Added helpers to the header src/gui/accountsettings.h: setEncryptionMessageIcon() and updateEncryptionMessageActions() and a QHash<QAction *, QToolButton *> _encryptionMessageButtons to track created buttons.
• Removed the previously-added useColoredBackground API from src/3rdparty/kmessagewidget/kmessagewidget.h/.cpp (restoring normal KMessageWidget interface/behavior) and cleaned up related styling changes, leaving KMessageWidget unchanged behavior-wise for other usages.
• Updated relevant includes and adjusted places where the old KMessageWidget text/icon/methods were referenced to use the new label/icon/button layout instead.

Testing

• No automated tests were executed for this change.

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Codex Task

[Rello]

internal test PR