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/search/+page.svelte

@@ -180,7 +180,11 @@
         {#await data.results}
             <SearchResultsSkeleton />
         {:then r}
-            {#if r.results.length === 0}
+            {#if r.error}
+                <p class="error" role="alert" data-testid="search-results-error">
+                    Couldn't load results: {r.error}
+                </p>
+            {:else if r.results.length === 0}
                 <p class="hint" data-testid="search-content-empty">
                     No pages match this search.
                 </p>
@@ -288,7 +292,11 @@
         {#await data.results}
             <SearchResultsSkeleton />
         {:then r}
-            {#if data.view === 'pages'}
+            {#if r.error}
+                <p class="error" role="alert" data-testid="search-results-error">
+                    Couldn't load results: {r.error}
+                </p>
+            {:else if data.view === 'pages'}
                 {#if r.pages.length === 0}
                     <p class="hint" data-testid="search-pages-empty">
                         No pages tagged with "{data.tag}".