[SideNav]: improvements

[ly-tempel-bitweb]

Problem

1. There are some accessibility violations:

2. Overlay colour doesn't match Figma. Should be background: var(--general-surface-overlay);.
3. Third Level Menu Items Parents Are Links adds a tree element to the parent when hovering or on a real mobile phone touching the "Treatments" or "Documentation" items.

Screen.Recording.2026-07-24.at.11.16.08.mov

4. According to this example in Figma, there should be a border-bottom between dropdown groups.

5. The size of the back icon should be 18px.

6. Add new prop collapsedText: an optional short-label field that falls back to the full text, keep the full text in the hover tooltip. Figma example with shorter texts when SideNav is collapsed.
7. Make stories full width on mobile.

8. Translate stories to Estonian.
9. Subitems don't have hover state.

Screen.Recording.2026-07-24.at.12.06.29.mov

10. Add collapsible WithGroupTitle example to showcase that the group title changes to a separator when SideNav is collapsed. Figma: https://www.figma.com/design/jWiRIXhHRxwVdMSimKX2FF/TEDI-READY-2.66.83?m=dev&node-id=17512-144755
11. Add props that allow consumer projects to override the sidenav.backToMainMenu and sidenav.backtoMenu texts.
12. Change tedi-sidenav__toggle-icon transition to transition: transform 0.2s ease-in-out; so it would match with CollapseButton.

Screen.Recording.2026-07-27.at.18.05.19.mov

13. Selected/active item shouldn't change colour on hover.
14. Some Storybook args are missing descriptions, for example SideNav.Dropdown args.