fix(frontend): make the hackathon manage sidebar flat, not collapsible

[hanaCasey]

Why

The Manage section of HackathonSidebar had grown a fold: Manage Hackathon
stayed on the rail and the rest of the organiser screens were disclosed from
a chevron beside it (7605a04, df8e8e9). But an $effect force-opens the
fold the moment you're anywhere inside the section (insideManage), and
every /manage/* route counts as inside it — so navigating to any manage
screen always rendered the full list anyway. That full list, inside a
sidebar with a fixed md:h-[calc(100vh-3.5rem)], is taller than the
viewport, and the fold bought nothing on the one set of pages an organiser
actually visits: it just forces overflow-y-auto to kick in on every manage
screen.

What changed

components/frontend/src/lib/components/layout/HackathonSidebar.svelte
only. Reverted the Manage section back to what it was before the fold: a
flat list of manageNav's items, always fully rendered.

• Dropped manageOpen/insideManage state, the manageHubItem/
  manageSubItems split, toggleManage, and the matching localStorage
  read/write.
• The <SidebarNavSection> call for Manage now passes items={manageItems}
  directly instead of parentItem/open/onToggle.
• SidebarNavSection.svelte itself is untouched — it already renders a
  plain list when no parentItem is passed, so no shared-component changes
  were needed.
• Left the unrelated --consent-banner-h padding and the nav's
  overflow-y-auto alone: that's a real, separate fix for the session-replay
  consent banner covering the last few sidebar entries, and it's the safety
  net for genuinely short viewports.

HackathonSidebar.test.ts rewritten for the new (non-)behavior: every
Manage entry renders on every page, there's no fold control, and a stale
assertion for "New Phase" (a manage-hub tile, never a sidebar entry) was
dropped from the "keeps every entry this branch has that main does not"
list — it already failed identically on unmodified develop.

Test plan

• vitest run on HackathonSidebar.test.ts + SidebarNavSection.test.ts — 24/24 pass.
• Full frontend vitest run — same 5 files / 6 tests fail as on unmodified origin/develop (confirmed by stashing this diff and re-running); no new failures introduced.
• svelte-check — not run in this environment (tool timed out/was interrupted repeatedly); no type-level changes were made, only state/prop removal already exercised by the updated tests.
• Reviewer: load /my/hackathon/<id>/manage as an owner and confirm the sidebar no longer needs to scroll for a normal item count.