Upgrade .NET Core 3.1 → .NET 8, React 16 → 18

[Copilot]

Full stack upgrade from EOL frameworks to current LTS versions.

Backend (.NET)

• netcoreapp3.1 → net8.0, enable nullable reference types, implicit usings
• Minimal hosting model — merge Startup.cs into Program.cs
• Replace deprecated WebClient with async HttpClient:

  // Before (synchronous, deprecated)
  using(var webClient = new WebClient()) {
      bytes = webClient.DownloadData(path);
  }
  // After
  bytes = await _httpClient.GetByteArrayAsync(path);

• NuGet upgrades: HtmlAgilityPack 1.12.4, Newtonsoft.Json 13.0.4 (fixes CVE GHSA-5crp-9r3c-p9vr), Swashbuckle 6.9.0
• Remove unused EPPlus dependency
• Fix NullReferenceException — storyNodes iterated without null check
• Add bounds checking in ParseCombatSkill/ParseEndurance to prevent IndexOutOfRangeException
• File-scoped namespaces, collection expressions, target-typed new

Frontend (React/TypeScript)

• React 16 → 18 with createRoot API
• Replace deprecated react-scripts-ts with react-scripts 5.0.1
• Add tsconfig.json, replace any types with proper interfaces (Response, StoryState, ChapterData, etc.)
• Convert class component wrappers to direct functional component exports

Original prompt

Please upgrade this application as much as possible.

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