fix: dashboard sidebar overlaps the footer when scrolling to the bottom

[thnndnly]

Summary

The dashboard side navigation (Startseite, Freiwillige, …) is position: fixed
and overlaps the footer when the page is scrolled to the bottom, covering the
left part of the footer (the "Träger"/ClubDialog logo area).

Steps to Reproduce

1. Log in, open any dashboard page.
2. Scroll all the way down to the footer.

Expected Behavior

The sidebar stops above the footer and stays within the page content; it should
not render on top of the footer.

Actual Behavior

The fixed sidebar (top: 112px, no bottom bound, z-index: 1) stays pinned to the
viewport and covers the left portion of the footer.

Technical notes

• Sidebar: src/components/Layout/DashboardLayout/NavigationBar.tsx
  (BarContainer: position: fixed; top: 112px; left: 0; z-index: 1; with no
  bottom/max-height).
• The horizontal overlap with content was already solved in left menu blocking the dashboard view #324 via
  margin-left on DashboardBaseContainer (styled/container.tsx). The footer
  lives outside that container (PageLayout → FooterPartnersSection, full
  width), so it is not protected — this is the vertical counterpart of left menu blocking the dashboard view #324.

Proposed fix (pick one)

• A (clean, no JS): convert the sidebar to position: sticky inside a real
  two-column flex layout; a sticky sidebar stops at the end of its column (above
  the footer). Moderate refactor of DashboardLayout/PageLayout.
• B (JS): IntersectionObserver on the footer, switch the sidebar from
  fixed to absolute when the footer enters view.
• C (quick, not exactly the desired behavior): give the footer a higher
  z-index + background so it covers the sidebar instead.