WebViews: clear loading indicator on main-frame load errors

[jim-daf]

Closes #555.

DocumentationWebView, ResponseBrowser and SinglePageBrowser all show a loading spinner that is hidden from inside onPageFinished (directly or via the onLoaded callback). None of them override onReceivedError, so a main-frame load failure leaves the spinner spinning forever:

showLoading()/isLoading = true
   -> network/cache error
   -> onPageFinished never fires
   -> hideLoading()/onLoaded() never runs

This is most reachable for the documentation view, which is configured with blockNetworkLoads = true and cacheMode = LOAD_CACHE_ELSE_NETWORK. The WebView is told to use only the cache, so a cache miss for the bundled documentation has no network fallback and the user is stuck on the spinner.

This PR adds onReceivedError(view, request, error) to each of the three WebViewClients and calls the same "loading is done" hook each one already uses:

override fun onReceivedError(view: WebView, request: WebResourceRequest, error: WebResourceError) {
    super.onReceivedError(view, request, error)
    if (request.isForMainFrame) {
        hideLoading() // or onLoaded(), depending on file
    }
}

Notes:

• request.isForMainFrame is the guard so sub-resource errors are still ignored. The existing onPageFinished path is unchanged for successful loads.
• One new import per file (android.webkit.WebResourceError). No new permission, no new resource, no new dependency.
• Total diff is 24 lines added across 3 files.