Fix SE4 changelog URL casing

[Ironship]

Summary

• Rename root ChangeLog.txt to Changelog.txt so the hard-coded SE 4 update-check URL resolves again.
• Keep the SE 5 update-check code unchanged; current SE 5 still tries change-log.txt first.
• Point generated UI release notes at the existing canonical change-log.txt URL and update the solution item.

Fixes #10540.

Verification

• Confirmed SE 4.0.15 uses https://raw.githubusercontent.com/SubtitleEdit/subtitleedit/main/Changelog.txt for update checks.
• Confirmed on the PR branch:
  • change-log.txt raw URL returns 200 (SE 5 primary URL)
  • Changelog.txt raw URL returns 200 (SE 4 hard-coded URL)
  • ChangeLog.txt raw URL returns 404 after the rename
• dotnet build .\src\ui\UI.csproj -c Debug --no-restore succeeded with 3 existing nullability warnings in unrelated export/batch-convert files.
• dotnet test .\tests\UI\UITests.csproj -c Debug --no-restore -v minimal passed: 124/124.

Notes

• This should not break current SE 5 beta update checks because change-log.txt remains available and is the first URL used by the SE 5 checker.
• Preserving all exact-case root filenames (ChangeLog.txt and Changelog.txt) would be the broadest URL compatibility option, but it would require tracking case-only duplicate files, which is risky for Windows contributors.
• AI assistance was used to identify, implement, and verify this change.

[Copilot]

Pull request overview

Fixes update-check failures for legacy Subtitle Edit 4.x clients by ensuring the repository-hosted changelog filename matches the hard-coded URL casing, and removes an obsolete fallback URL from the SE 5 update checker.

Changes:

• Added/renamed the root changelog to Changelog.txt (capital C, rest lower) so SE4’s update-check URL resolves again.
• Updated SE5 update-check logic to remove the now-dead ChangeLog.txt fallback URL.
• Updated solution/workflow references to point at the correctly cased changelog filename.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/ui/Features/Help/CheckForUpdates/CheckForUpdatesViewModel.cs	Removes ChangeLog.txt fallback URL and returns “unable to check” when both remaining URLs fail.
SubtitleEdit.sln	Updates Solution Items entry from ChangeLog.txt to Changelog.txt.
Changelog.txt	Introduces the correctly cased changelog file at repo root for SE4 update checks.
.github/workflows/build-ui.yml	Updates release-notes “Change Log” link to Changelog.txt.

[niksedk]

I will merge this in a week or two, as this will break the SE 5 beta updates for versions before beta21.
This will also make SE 4 update check work again.

[InsideTheMindofThePuzzledOne]

Thank you niksedk it is much appreciated and such a great program.