Add embed:// scheme for serving trusted local content

[maxim-productengine]

Added a new custom embed:// scheme served entirely by Dullahan from an on-disk root that the Viewer configures. Content is treated by CEF as a proper secure origin.

Registry allowlist. The host configures a registry of allowed paths. Exact-match paths are supported for individual files; entries ending in / are treated as directory prefixes. Any request that doesn't match returns 404 without touching disk.
Related Viewer-side API: setEmbedSchemeRoot(root_dir) and setEmbedRegistry(allowed_paths) to configure the scheme handler.

Bridge scoping. The window.JSONtoCPP JS bridge is now injected only on frames whose URL is under embed://. Ordinary web pages don't see the bridge. Each JS→C++ message also carries the calling frame's URL so the host can verify origin.

Recursive-inheritance enforcement. A browser whose top-level document is an embed:// page may only load embed:// or data: sub-resources - no external iframes, images, scripts, stylesheets, fetches, or beacons. Conversely, a browser NOT running embed:// content cannot fetch embed:// resources. Applied via OnBeforeResourceLoad; enforced by tracking the trust context set in OnBeforeBrowse to avoid a UI/IO thread race that would otherwise misclassify sub-resources fired right after a navigation commits.

Navigation lockdown. Top-level navigation into embed:// from a user-clicked link or an HTTP redirect is rejected. Loading embed:// requires an explicit LoadURL call from the host.

[callumlinden]

Looks great - nice work. Just a couple of minor comments:

• I see references to the scheme name and format in many places, in different files. E.G.

  const char* kEmbedScheme = "embed";
  const char* kEmbedSchemePrefix = "embed://";
  

• Would it be worth centralizing that in a header perhaps so that it only needs to be changed once?

• See a lot of places where the lowercase version of a string is created. Is that okay for non-ascii strings (I think it is - just checking) and worth using a single 'canonical' ToLower() function like:

  std::string text = "HeLLo WoRLd!";
  auto lower_text = text
                | std::views::transform([](unsigned char c) { return std::tolower(c); })
                | std::ranges::to<std::string>(); // Converts view back to std::string
  std::cout << lower_text; // outputs: hello world!
  

• Might need to add entries in the mime_for_path() function for other file types in Garfield's 3p package - E.G. TypeScript .ts

[maxim-productengine]

Thanks, it makes sense.
I'll add one more commit with the changes you suggested before merging.