Add Prep mode naming and presentation for preparation workflows

[KSemenenko]

Summary

Add Prep mode naming or presentation for workflows primarily used for scanning and preparation rather than rote memorization. Treat this as additive unless a later task explicitly confirms full Learn-mode renaming/removal.

Source

• Category: Teleprompter & Prep
• Feedback video: https://youtu.be/IeUoMyx7dKE?t=30m42s

Acceptance criteria

• Preparation-focused entry points can present Prep naming or an equivalent preparation view.
• Existing Learn behavior remains available unless explicitly removed later.
• Navigation, buttons, tooltips, tests, and localization use consistent wording for the new presentation.
• Add or update browser/UI test coverage for the relevant user flow.
• Solution tests pass before the issue is marked ready for user verification.

Implementation note

Treat this as an additive feature or presentation option. Do not remove existing behavior unless the user explicitly confirms deletion.

[KSemenenko]

Implemented additive Prep mode presentation in commit 78688b1.\n\nWhat changed:\n- Added /prep and /prep?id=... routes that reuse the existing Learn/RSVP preparation workflow without removing /learn.\n- Added a localized Prep library card action and Prep navigation suggestion for AI Spotlight.\n- Kept existing Practice/Learn and Teleprompter actions available.\n- Added UI coverage for the Prep library launch flow, route readiness, back navigation, and continued /learn availability.\n\nVerification passed:\n- dotnet format ./PrompterOne.slnx --verify-no-changes --no-restore --include \n- dotnet build ./PrompterOne.slnx -warnaserror --no-restore\n- dotnet test --project tests/PrompterOne.Web.Tests/PrompterOne.Web.Tests.csproj --no-build: 627/627\n- dotnet test --project tests/PrompterOne.Web.UITests.Shell/PrompterOne.Web.UITests.Shell.csproj --no-build: 63/63\n- dotnet test @./tests/dotnet-test-progress.rsp --solution ./PrompterOne.slnx: 1185/1185\n\nLeaving this open for your review/verification.