Improve installers catalog and update flow

[cytrafps]

Summary

• redesign the Installers tab around a curated 51-app catalog with instant search and category filters
• add Essentials selection, a visible sequential install queue, individual install/update actions, retry support, and clearer WinGet errors
• detect installed apps and available updates while avoiding unauthenticated GitHub REST API rate limits
• add official publisher links, manual setup handling for community modifications, and 50 packaged SVG brand assets
• retain Microsoft Store search with improved result metadata and safer download/install handling

Why

The previous page focused on Microsoft Store search and did not provide a guided way to install common desktop applications. Update checks also depended on unauthenticated GitHub REST requests, which could exhaust the shared hourly limit and mark every installed application as unchecked.

This change provides a cleaner first-run experience while keeping installs tied to Microsoft WinGet manifests or official publisher instructions. Downloaded fallback installers are checked against the SHA-256 value from their manifest.

Validation

• dotnet build SynToolkit/SynToolkit.csproj --no-restore succeeds with 0 errors
• all non-invasive SynToolkit.SystemInformationTests pass
• all 51 catalog icon references resolve to 50 packaged SVG files; Vencord and Vesktop intentionally share one asset
• all SVG assets parse successfully

The build still reports the two pre-existing nullable warnings in WindowsWallpaperService.cs; this PR does not modify that file.

[cytrafps]

valid