Add proxy support and configurable download location

[siator72]

Changes

Proxy support (new feature)

• IProxyService, ProxyService, ProxyEntry - manage multiple proxy entries with runtime switching
• DownloadLocationService - configurable download location
• Settings page UI for proxy & download location management (SettingsPage.xaml / .cs, SettingsViewModel)
• New localized strings in Resources.resw
• Wired up in App.xaml.cs, ActivationService, DownloadManagerService, GitHubUpdaterService, DownloadHelper

StoreListings dependency

This feature needs the proxy-routing changes in the StoreListings submodule (ProxyManager + Helpers.cs). Those are submitted separately as mjishnu/StoreListings#1. .gitmodules is untouched - once that PR is merged, this PR's submodule pointer can be bumped to the upstream commit and everything stays on mjishnu/StoreListings. Until then, this branch points the submodule at my fork commit (ddf5abf) so it remains buildable/testable.

All changes built and tested locally (WinUI 3 / .NET 10, x64).

[mjishnu]

I merged the store‑listing PR after making some edits. Specifically, I switched from destroying and rebuilding HttpClient instances to using an IWebProxy wrapper, which is thread‑safe.

This requires a small change in the Raven PR:

StoreListings.Library.ProxyManager.Proxy = webProxy;

instead of

StoreListings.Library.Internal.ProxyManager.SetProxy(webProxy);