Polish Web GUI uploads

[rndv1]

Summary

• replace the plain screenshot file input with a drag-and-drop dropzone
• show the selected file name in the upload control
• keep the native file input as the accessible fallback
• 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://127.0.0.1:5299
• verified GET / returns 200, dropzone markup is rendered, and upload.js is referenced
• verified multipart POST still generates a data PNG, download link, success message, and 1080x1080 metadata
• captured a headless Edge screenshot of the updated Web GUI
• git diff --check origin/main...HEAD