Add Web GUI session history

[rndv1]

Summary

• store recent generated cards in the current SnapForge.Web session
• show a session history list with title, time, preset, theme, size, and PNG download links
• keep only the latest 5 generated cards in memory-backed session storage
• update the README feature list and roadmap

Verification

• dotnet format --verify-no-changes --no-restore
• dotnet build --no-restore --configuration Release
• dotnet test --no-build --configuration Release --verbosity normal
• dotnet pack src/SnapForge.Cli/SnapForge.Cli.csproj --no-build --configuration Release --output artifacts/packages
• ran SnapForge.Web locally on http://127.0.0.1:5300
• verified two multipart POST generations in one WebRequestSession render two history entries and two history download links
• captured a headless Edge screenshot of the Web GUI start screen
• git diff --check origin/main...HEAD