Gracefully handle upstream fetch failures for release lists and diffs

[kelset]

Summary

While doing a maintenance pass on the repo and triaging #411, I found that the original outage is no longer reproducible, but the investigation surfaced a real resilience gap in the app.

Today the UI assumes upstream GitHub fetches succeed and return valid content. When they do not (for example due to a transient GitHub/raw outage, a 404/429 response, or some other unexpected payload), the app can blank-screen, throw semver errors, or fail without a clear recovery path.

Concrete problem areas

src/hooks/fetch-release-versions.ts

• fetches RELEASES from upstream
• does not check response.ok
• blindly parses response.text().split('\n')

src/components/common/VersionSelector.tsx

• assumes fetched release entries are valid semver values
• can throw if upstream returns unexpected content such as 404: Not Found
• one example reported in Site not working as expected #411 was Invalid Version: 404: Not Found

src/hooks/fetch-diff.ts

• fetches the diff file from upstream
• does not check response.ok
• passes the response body directly to parseDiff(...)
• does not surface a user-visible error state when the request fails

Desired behavior

• Guard upstream fetches with response.ok
• Handle network failures and rejected fetches explicitly
• Validate upstream release payloads before running semver operations
• Show a user-visible error state instead of blank-screening or failing silently
• Keep the page usable so users can change versions and retry
• Where useful, distinguish between:
  • upstream/network failure
  • selected diff not existing upstream
  • invalid/unexpected upstream payload

Validation ideas

• Add tests for failed RELEASES fetches
• Add tests for invalid release payloads
• Add tests for failed diff fetches
• Verify the app no longer throws or blank-screens when upstream returns an error body

Context

This is split out from #411 so that the mixed outage report there can be closed and any remaining actionable work can be tracked in a more focused issue.