Add Web GUI foundation

[rndv1]

Summary

• add SnapForge.Web as an ASP.NET Core Razor Pages app
• implement a local browser card generator backed by SnapForge.Core
• support screenshot upload, title/subtitle inputs, preset/theme selection, preview, and PNG download
• add a generated sample preview asset for the first screen
• document Web GUI usage and contribution checks

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://127.0.0.1:5298
• verified GET / returns 200
• verified multipart POST upload generates a data PNG, download link, success message, and 1080x1080 metadata
• captured a headless Edge screenshot of the Web GUI start screen
• git diff --check origin/main...HEAD