feat: revamp demo application with interactive playground

- Move demo app from BlazorTextDiff.Web/ to src/BlazorTextDiff.Web/
- Replace 5 separate demo pages with 3 focused pages:
  - Index.razor: interactive playground with toolbar toggles
    (collapse/case/whitespace), real-time diff editing
  - CharacterHighlight.razor: typo correction and code refactoring examples
  - Async.razor: async loading demo from GitHub
- Remove IgnoreCaseDemo.razor, IgnoreWhitespaceDemo.razor, SurveyPrompt.razor
- Update NavMenu with 3 entries (Home, Characters, Async)
- Improve loading spinner with SVG animation
- Better error UI in index.html
- Remove unused HttpClient and weather forecast services
- Enable AOT compilation in csproj

Author: lzinga