feat(snapshots): Add frontend logic for downloading images zip

[NicoHinderling]

Screenshots:

Summary

Frontend companion to #115337. Adds a "Download Images" action to the snapshot header dropdown menu that fetches the zip from the new download endpoint and triggers a browser download.

Also cleans up the SnapshotImage type and test fixtures:

• Removes content_hash from the frontend SnapshotImage interface — it's an internal storage detail not needed by the UI
• Stops filtering content_hash from the metadata copy-to-clipboard JSON (since the field is gone)
• Filters out null values from the metadata JSON to reduce noise
• Reorders dropdown menu items: Build Metadata → View Base Build → Download Images → Rerun Status Checks → Delete

Test plan

• Open a snapshot detail page and verify the dropdown menu shows the new "Download Images" option
• Click "Download Images" and confirm the zip downloads with the correct filename
• Verify the metadata copy button no longer includes content_hash or null fields
• Confirm existing snapshot stories still render correctly without content_hash

[NicoHinderling]

• feat(snapshots): Add frontend logic for downloading images zip #115338 👈 (View in Graphite)
• feat(snapshots): Add endpoint for downloading images zip #115337
• master

This stack of pull requests is managed by Graphite. Learn more about stacking.

[github-actions]

📊 Type Coverage Diff

Metric	Before	After	Delta
Coverage	93.47%	93.47%	±0%
Typed	134,857	134,863	🟢 +6
Untyped	9,424	9,425	🔴 +1

🔍 1 new type safety issue introduced

any-typed symbols (1 new)

File	Line	Detail
static/app/views/preprod/snapshots/header/snapshotHeaderActions.tsx	172	detail (var)

This is informational only and does not block the PR.

[cursor]

Cursor Bugbot has reviewed your changes and found 2 potential issues.

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

^{Reviewed by Cursor Bugbot for commit 644152b. Configure here.}