image-cache: use custom URI scheme for transparent cache-miss re-download

Instead of rewriting img src to file:// URIs, rewrite to a custom
feedthemonkey-img:/// scheme. A WebKit URI scheme handler is registered
on the WebView's WebContext that:

- Serves the image directly from the cache directory if present.
- On a cache miss (e.g. after the user deletes ~/.cache), spawns a
  reqwest download in the tokio runtime, then resumes on the GLib main
  loop via glib::spawn_future_local and serves the freshly downloaded
  bytes — all transparent to the WebView.

This means deleting the cache directory never results in permanently
broken images; they are silently re-fetched on first access.

src/window.rs

@@ -245,6 +245,9 @@ pub mod imp {
 
         fn setup_webview(&self) {
             let wv = &*self.web_view;
+            if let Some(ctx) = wv.web_context() {
+                crate::image_cache::register_scheme(&ctx);
+            }
 
             // Load content.html from GResource, inlining the CSS so WebKit
             // doesn't need to fetch it over a custom scheme.