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/main/runtime/anime-browser-runtime.ts

@@ -389,6 +389,7 @@ export function createAnimeBrowserRuntime(deps: AnimeBrowserRuntimeDeps) {
           version: extension.version,
           nsfw: extension.nsfw,
           repoUrl: extension.repoUrl,
+          iconUrl: extension.iconUrl,
           sourceNames: extension.sourceNames,
           installed: installedPkgs.has(extension.pkg),
         })),