perf(preprod): Virtualize snapshot sidebar for 40k image builds

[NicoHinderling]

The snapshot sidebar rendered every item as a real DOM node. When viewing
a build with 40,000 images, this meant 40k+ DOM elements in the sidebar
alone, causing severe page lag — scrolling, typing in search, and
navigating were all sluggish.

This replaces the flat rendering with @tanstack/react-virtual (the
same library already used by the main list view). Only ~50 sidebar nodes
exist in the DOM at any time (visible items + 25 overscan). The section
header collapse/expand, search filtering, active-item highlighting, and
scroll-to-active behaviors are all preserved.

Additional changes:

• Added loading="lazy" and decoding="async" to the LazyImage
  component's <img> as a belt-and-suspenders measure alongside the
  existing virtualizer
• Replaced <Disclosure> section headers with a standalone button
  (Disclosure wraps children, which is incompatible with virtualization),
  preserving aria-expanded for accessibility
• Memoized SidebarItem and SectionHeaderRow sub-components
• Fixed scroll-to-active to only fire on actual activeItemKey changes,
  not on collapse/expand which also mutates virtualRows

[github-actions]

📊 Type Coverage Diff

Metric	Before	After	Delta
Coverage	93.56%	93.56%	±0%
Typed	135,578	135,596	🟢 +18
Untyped	9,334	9,336	🔴 +2

🔍 2 new type safety issues introduced

Non-null assertions (!) (2 new)

File	Line	Detail
static/app/views/preprod/snapshots/sidebar/snapshotSidebarContent.tsx	137	virtualRows[i]!
static/app/views/preprod/snapshots/sidebar/snapshotSidebarContent.tsx	210	virtualRows[vi.index]!

This is informational only and does not block the PR.

[mtopo27]

fix snapshots

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 51c6d93. Configure here.}