Add keyboard shortcuts for reader

Summary

Implement keyboard shortcuts for controlling the reader.

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.

@byskyphy

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.