Export script as .md or .txt

[KSemenenko]

Summary

Add buttons to easily export the script as an .md or .txt file.

Source

• Category: Recording & Exporting
• Feedback video: https://youtu.be/IeUoMyx7dKE?t=42m16s

Acceptance criteria

• Users can export the current script as Markdown (.md).
• Users can export the current script as plain text (.txt).
• Export actions use PrompterOne's Import/Export wording consistently.
• 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]

Implemented in 0546b80 and pushed to main.\n\nScope:\n- Added separate editor header buttons for Export .md and Export .txt.\n- Kept native export available through the existing Export action.\n- Export .md and .txt write the current editor source with the expected .md/.txt filenames.\n- Updated export wording and localized the new labels across supported resource files.\n- Kept small-screen header controls reachable by prioritizing Learn/Read before export actions in mobile header order.\n\nVerification:\n- dotnet build ./PrompterOne.slnx -warnaserror --no-restore\n- dotnet test --project tests/PrompterOne.Web.Tests/PrompterOne.Web.Tests.csproj --no-build -- --no-ansi --no-progress (619/619)\n- dotnet test --project tests/PrompterOne.Web.UITests.Editor/PrompterOne.Web.UITests.Editor.csproj --no-build -- --no-ansi --no-progress (155/155)\n- dotnet test --project tests/PrompterOne.Web.UITests.Shell/PrompterOne.Web.UITests.Shell.csproj --no-build -- --no-ansi --no-progress (62/62)\n- dotnet test --project tests/PrompterOne.Web.UITests.Reader/PrompterOne.Web.UITests.Reader.csproj --no-build -- --no-ansi --no-progress (169/169)\n- dotnet test @./tests/dotnet-test-progress.rsp --solution ./PrompterOne.slnx (1168/1168)\n\nReady for review @byskyphy.