feat(anime): show extension and repo icons in extensions panel

- Add extension-icons module (buildIconIndex, isSafeIconUrl, repoFaviconUrl, iconMonogram) with tests
- Render each row's icon with a monogram fallback while loading or missing
- Installed extensions borrow their icon from the catalogue; repo rows use the index host's favicon
- Restrict icon loads to https URLs
- Thread iconUrl through AvailableExtension and the runtime, document the feature

src/types/anime-browser.ts

@@ -156,6 +156,8 @@ export interface AvailableExtension {
   version: string;
   nsfw: boolean;
   repoUrl: string;
+  /** Where the repository publishes the extension's icon; may 404. */
+  iconUrl: string;
   sourceNames: string[];
   installed: boolean;
 }