Add keyboard shortcuts for reader

[KSemenenko]

Summary

Implement keyboard shortcuts for controlling the reader.

Source

• Category: Teleprompter & Prep
• Feedback video: https://youtu.be/IeUoMyx7dKE?t=32m57s

Acceptance criteria

• Reader controls support documented keyboard shortcuts on PrompterOne browser surfaces.
• Shortcuts do not conflict with text editing inputs or browser accessibility expectations.
• 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 2d0ebca and pushed to main.

What changed:

• Expanded Learn reader browser coverage for Space, L, ArrowUp/ArrowDown, ArrowLeft/ArrowRight, and PageUp/PageDown shortcuts.
• Expanded Teleprompter browser coverage for Space, block navigation, orientation, horizontal/vertical mirroring, and alignment shortcuts.
• Added a focused Teleprompter range-input test proving focused reader controls do not let page-level shortcuts fire.

Verification:

• dotnet format ./PrompterOne.slnx --verify-no-changes --no-restore --include tests/PrompterOne.Web.UITests/Support/BrowserTestConstants.cs tests/PrompterOne.Web.UITests.Reader/Learn/LearnKeyboardShortcutFlowTests.cs tests/PrompterOne.Web.UITests.Reader/Teleprompter/TeleprompterKeyboardShortcutFlowTests.cs
• dotnet build ./PrompterOne.slnx -warnaserror --no-restore
• dotnet test @./tests/dotnet-test-progress.rsp --project ./tests/PrompterOne.Web.UITests.Reader/PrompterOne.Web.UITests.Reader.csproj -- --treenode-filter "///LearnKeyboardShortcutFlowTests/*"
• dotnet test @./tests/dotnet-test-progress.rsp --project ./tests/PrompterOne.Web.UITests.Reader/PrompterOne.Web.UITests.Reader.csproj -- --treenode-filter "///TeleprompterKeyboardShortcutFlowTests/*"
• dotnet test @./tests/dotnet-test-progress.rsp --project ./tests/PrompterOne.Web.UITests.Reader/PrompterOne.Web.UITests.Reader.csproj (172/172 passed)
• dotnet test @./tests/dotnet-test-progress.rsp --solution ./PrompterOne.slnx (1181/1181 passed)

@byskyphy please review when ready.