fix: surface streamed load failures inline

The streamed loaders on bookmarks, collections, library, search, and the
collection detail page swallowed only ApiError into their in-band `error`
field and re-threw anything else. A raw network failure (a non-ApiError
TypeError from fetch) therefore escaped the `{#await}` to SvelteKit's
generic error page. On the collection detail page a content-load failure
showed a misleading "this collection is empty", and on search a failed
results query showed a false "no matches" because the streamed `error`
was never rendered.

Fold non-ApiError failures into the same in-band `error` so every
transient blip renders as an inline message, add a `contentError` branch
to the collection detail grid, and render the search results `error` in
both result views.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/routes/collections/+page.ts

@@ -16,10 +16,11 @@ export const load: PageLoad = async () => {
                 if (e instanceof ApiError && e.status === 401) {
                     return { collections: [], authenticated: false, error: null as string | null };
                 }
-                if (e instanceof ApiError) {
-                    return { collections: [], authenticated: true, error: e.message };
-                }
-                throw e;
+                // An HTTP error or a raw network failure (a non-ApiError
+                // TypeError) renders inline rather than escaping to the
+                // framework error page for a transient API blip.
+                const message = e instanceof ApiError ? e.message : 'Something went wrong.';
+                return { collections: [], authenticated: true, error: message };
             }
         })()
     };

frontend/src/routes/collections/[id]/+page.svelte

@@ -29,6 +29,9 @@
     let mangas = $state<Manga[]>([]);
     let pages = $state<CollectionPageItem[]>([]);
     let contentLoading = $state(true);
+    // Set when the streamed content fails to load, so we surface an inline
+    // error instead of a misleading "this collection is empty".
+    let contentError = $state<string | null>(null);
 
     // Seed (and re-seed on navigation) from the streamed content. A monotonic
     // guard drops a stale resolution that lands after a newer navigation.
@@ -37,6 +40,7 @@
         const content = data.content;
         const seq = ++contentSeq;
         contentLoading = true;
+        contentError = null;
         content
             .then((r) => {
                 if (seq !== contentSeq) return;
@@ -44,8 +48,11 @@
                 pages = r.pages;
                 contentLoading = false;
             })
-            .catch(() => {
+            .catch((e) => {
                 if (seq !== contentSeq) return;
+                mangas = [];
+                pages = [];
+                contentError = e instanceof Error ? e.message : 'Something went wrong.';
                 contentLoading = false;
             });
     });
@@ -203,6 +210,10 @@
 
 {#if contentLoading}
     <MangaGridSkeleton count={6} />
+{:else if contentError}
+    <p class="error" role="alert" data-testid="collection-content-error">
+        Couldn't load this collection's contents: {contentError}
+    </p>
 {:else if mangas.length === 0 && pages.length === 0}
     <p class="status" data-testid="collection-empty">
         This collection is empty.