enhancement: add focus indicator to hamburger menu button

[Killian-Aidalinfo]

📝 Description

This change adds a visible focus indicator to the hamburger menu button (SidebarToggler) to improve keyboard accessibility. It ensures that when users navigate using the Tab key, the button receives a distinct outline, making it easier to identify which element is currently focused.

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🐛 Addressed Issue

Issues 1202

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🔧 Additional Details

Added the following CSS rule to apply a visible focus indicator:

.navbar-toggler:focus,
.sidebar-toggler:focus,
.c-header-toggler:focus {
  outline: 2px solid $light-blue !important;
  outline-offset: 2px;
}

This helps meet accessibility standards and provides a better experience for keyboard-only users.

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Checklist

• I have read and understand the contributing guidelines
• This PR introduces new or alters existing behavior, and I have updated the documentation accordingly