feat(#5099): implement collapsible sidebar functionality

[SteKoe]

This pull request introduces a new collapsible sidebar for the InstanceShell view, streamlining and modernizing the navigation experience. The sidebar can now be toggled open or closed, with its state persisted in local storage. Related code and style changes were made to support this feature, and some legacy sidebar logic was removed for clarity and maintainability.

Sidebar Refactor and Collapsible Functionality:

• Added a collapsible sidebar to InstanceShell.vue, including a toggle button, dynamic width, and persistent open/close state using local storage. The sidebar's visibility and width now respond to user interaction, improving usability. [1] [2] [3]
• Refactored sidebar.vue to remove old sidebar open/close logic and classes, delegating sidebar state control to the parent component. Simplified sidebar rendering and removed unused toggle code. [1] [2] [3] [4] [5] [6] [7]

UI and Layout Adjustments:

• Adjusted the main layout in InstanceShell.vue to use a flex layout, ensuring the sidebar and main content area are displayed side by side and resize appropriately with the sidebar state.
• Updated the sticky sub-navigation bar to always stick to the very top of the page, improving consistency in navigation.
• Simplified button and sticky directive styles for better alignment and positioning. [1] [2]