Add option to center text

[KSemenenko]

Summary

Include an option to center the text in the editor/viewer.

Source

• Category: Script Editing
• Feedback video: https://youtu.be/IeUoMyx7dKE?t=28m51s

Acceptance criteria

• Users can choose centered text alignment in the relevant editor/viewer surface.
• The option coexists with existing alignment behavior and persists where expected.
• Add or update browser/UI test coverage for the relevant user flow.
• Solution tests pass before the issue is marked ready for user verification.

[KSemenenko]

Added issue-specific centered text alignment coverage in commit 56dd322.\n\nNotes:\n- The teleprompter/viewer production UI already exposes left, center, right, and justify alignment controls.\n- This commit adds a dedicated browser persistence regression for the Center alignment option: click Center, verify the reader surface reports center alignment, verify local storage stores ReaderTextAlignment.Center, reload, and verify center alignment remains active.\n\nVerification passed:\n- dotnet format ./PrompterOne.slnx --verify-no-changes --no-restore --include tests/PrompterOne.Web.UITests.Reader/Teleprompter/TeleprompterPersistenceTests.cs\n- dotnet build ./PrompterOne.slnx -warnaserror --no-restore\n- focused center persistence browser test: 1/1\n- dotnet test --project tests/PrompterOne.Web.UITests.Reader/PrompterOne.Web.UITests.Reader.csproj --no-build: 175/175\n- dotnet test @./tests/dotnet-test-progress.rsp --solution ./PrompterOne.slnx: 1187/1187\n\nLeaving this open for your review/verification.